<?xml version="1.0" encoding="UTF-8"?><edgarSubmission xmlns="http://www.sec.gov/edgar/nport" xmlns:com="http://www.sec.gov/edgar/common" xmlns:ncom="http://www.sec.gov/edgar/nportcommon" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sec.gov/edgar/nport eis_NPORT_Filer.xsd">
  <headerData>
    <submissionType>NPORT-P</submissionType>
    <isConfidential>false</isConfidential>
    <filerInfo>

      <filer>
        <issuerCredentials>
          <cik>0000783740</cik>
          <ccc>XXXXXXXX</ccc>
        </issuerCredentials>
      </filer>


      <seriesClassInfo>
        <seriesId>S000002506</seriesId>
        <classId>C000006845</classId>
        <classId>C000006838</classId>
        <classId>C000006843</classId>
        <classId>C000006849</classId>
        <classId>C000006847</classId>
        <classId>C000006844</classId>
        <classId>C000006850</classId>
        <classId>C000006839</classId>
        <classId>C000231058</classId>
      </seriesClassInfo>


    </filerInfo>
  </headerData>
  <formData>
    <genInfo>
      <regName>MFS SERIES TRUST X</regName>
      <regFileNumber>811-04492</regFileNumber>
      <regCik>0000783740</regCik>
      <regLei>5493001MUVNRNUPJY164</regLei>
      <regStreet1>111 HUNTINGTON AVENUE</regStreet1>
      <regCity>BOSTON</regCity>
      <regStateConditional regCountry="US" regState="US-MA"/>
      <regZipOrPostalCode>02199</regZipOrPostalCode>
      <regPhone>617-954-5000</regPhone>
      <seriesName>MFS Aggressive Growth Allocation Fund</seriesName>
      <seriesId>S000002506</seriesId>
      <seriesLei>549300UEFZYWREX0YF53</seriesLei>
      <repPdEnd>2026-05-31</repPdEnd>
      <repPdDate>2026-02-28</repPdDate>
      <isFinalFiling>N</isFinalFiling>
    </genInfo>
    <fundInfo>
      <totAssets>3074129764.10</totAssets>
      <totLiabs>2531555.81</totLiabs>
      <netAssets>3071598208.29</netAssets>
      <assetsAttrMiscSec>0.00000000</assetsAttrMiscSec>
      <assetsInvested>0.00000000</assetsInvested>
      <amtPayOneYrBanksBorr>0.00000000</amtPayOneYrBanksBorr>
      <amtPayOneYrCtrldComp>0.00000000</amtPayOneYrCtrldComp>
      <amtPayOneYrOthAffil>0.00000000</amtPayOneYrOthAffil>
      <amtPayOneYrOther>0.00000000</amtPayOneYrOther>
      <amtPayAftOneYrBanksBorr>0.00000000</amtPayAftOneYrBanksBorr>
      <amtPayAftOneYrCtrldComp>0.00000000</amtPayAftOneYrCtrldComp>
      <amtPayAftOneYrOthAffil>0.00000000</amtPayAftOneYrOthAffil>
      <amtPayAftOneYrOther>0.00000000</amtPayAftOneYrOther>
      <delayDeliv>0.00000000</delayDeliv>
      <standByCommit>0.00000000</standByCommit>
      <liquidPref>0.00000000</liquidPref>
      <cshNotRptdInCorD>0.00000000</cshNotRptdInCorD>
      <isNonCashCollateral>N</isNonCashCollateral>
      <returnInfo>
        <monthlyTotReturns>
          <monthlyTotReturn classId="C000006845" rtn1="0.48000000" rtn2="3.11000000" rtn3="2.03000000"/>
          <monthlyTotReturn classId="C000006838" rtn1="0.43000000" rtn2="3.10000000" rtn3="2.02000000"/>
          <monthlyTotReturn classId="C000006843" rtn1="0.35000000" rtn2="3.02000000" rtn3="1.98000000"/>
          <monthlyTotReturn classId="C000006849" rtn1="0.40000000" rtn2="3.09000000" rtn3="1.98000000"/>
          <monthlyTotReturn classId="C000006847" rtn1="0.40000000" rtn2="3.00000000" rtn3="1.98000000"/>
          <monthlyTotReturn classId="C000006844" rtn1="0.36000000" rtn2="3.05000000" rtn3="1.94000000"/>
          <monthlyTotReturn classId="C000006850" rtn1="0.42000000" rtn2="3.09000000" rtn3="2.01000000"/>
          <monthlyTotReturn classId="C000006839" rtn1="0.45000000" rtn2="3.11000000" rtn3="2.04000000"/>
          <monthlyTotReturn classId="C000231058" rtn1="0.47000000" rtn2="3.11000000" rtn3="2.06000000"/>
        </monthlyTotReturns>
        <othMon1 netRealizedGain="1482835.00000000" netUnrealizedAppr="-238842247.84000000"/>
        <othMon2 netRealizedGain="1081746.07000000" netUnrealizedAppr="91551020.31000000"/>
        <othMon3 netRealizedGain="1551926.81000000" netUnrealizedAppr="60213306.48000000"/>
      </returnInfo>
      <mon1Flow redemption="55445214.75000000" reinvestment="206094671.43000000" sales="39769059.46000000"/>
      <mon2Flow redemption="71151787.33000000" reinvestment="0.00000000" sales="40791101.32000000"/>
      <mon3Flow redemption="52451432.27000000" reinvestment="0.00000000" sales="30196847.80000000"/>



    </fundInfo>
    <invstOrSecs>
      <invstOrSec>
        <name>MFS Research International Fund</name>
        <lei>UH91KW1VFNZQF5FY7237</lei>
        <title>MFS Research International Fund</title>
        <cusip>55273H361</cusip>
        <identifiers>
          <isin value="US55273H3619"/>
        </identifiers>
        <balance>9088490.25000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>260294360.67000000</valUSD>
        <pctVal>8.474232077863</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Emerging Markets Equity Fund</name>
        <lei>GLINMRWNKCBOHYKQ2131</lei>
        <title>MFS Emerging Markets Equity Fund</title>
        <cusip>552746331</cusip>
        <identifiers>
          <isin value="US5527463310"/>
        </identifiers>
        <balance>1933027.73000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>96941340.66000000</valUSD>
        <pctVal>3.156055385055</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Growth Fund</name>
        <lei>K4ZYIH3WSXKUSPKIPM59</lei>
        <title>MFS Growth Fund</title>
        <cusip>552985673</cusip>
        <identifiers>
          <isin value="US5529856735"/>
        </identifiers>
        <balance>1687099.95000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>318861890.74000000</valUSD>
        <pctVal>10.38097658344</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International New Discovery Fund</name>
        <lei>RC53GXLQZ5LVIZBRHR66</lei>
        <title>MFS International New Discovery Fund</title>
        <cusip>552981383</cusip>
        <identifiers>
          <isin value="US5529813835"/>
        </identifiers>
        <balance>3530948.15000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>129268011.59000000</valUSD>
        <pctVal>4.208493521096</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS New Discovery Value Fund</name>
        <lei>LRMRW30Z8FYTIFW1W860</lei>
        <title>MFS New Discovery Value Fund</title>
        <cusip>552982696</cusip>
        <identifiers>
          <isin value="US5529826969"/>
        </identifiers>
        <balance>4244505.60000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>77292446.96000000</valUSD>
        <pctVal>2.516359293067</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International Intrinsic Value Fund</name>
        <lei>7540QFSKYNTDFH7HAK24</lei>
        <title>MFS International Intrinsic Value Fund</title>
        <cusip>552746349</cusip>
        <identifiers>
          <isin value="US5527463492"/>
        </identifiers>
        <balance>3354000.46000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>161696362.22000000</valUSD>
        <pctVal>5.264241976167</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Mid Cap Growth Fund</name>
        <lei>XPX0WXKIO6CLKBLYF666</lei>
        <title>MFS Mid Cap Growth Fund</title>
        <cusip>552987554</cusip>
        <identifiers>
          <isin value="US5529875545"/>
        </identifiers>
        <balance>10225549.61000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>290814630.97000000</valUSD>
        <pctVal>9.467860418238</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Global Real Estate Fund</name>
        <lei>ROYDM6SL56ZXYAENS860</lei>
        <title>MFS Global Real Estate Fund</title>
        <cusip>552982720</cusip>
        <identifiers>
          <isin value="US5529827207"/>
        </identifiers>
        <balance>9067092.85000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>157948757.46000000</valUSD>
        <pctVal>5.142233676061</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS New Discovery Fund</name>
        <lei>ZVEMRLDGMDO1D1VDSU87</lei>
        <title>MFS New Discovery Fund</title>
        <cusip>55273H346</cusip>
        <identifiers>
          <isin value="US55273H3460"/>
        </identifiers>
        <balance>2057611.59000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>75411464.81000000</valUSD>
        <pctVal>2.455121395971</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Institutional Money Market Portfolio</name>
        <lei>5L2W638H85N8T1136O07</lei>
        <title>MFS Institutional Money Market Portfolio</title>
        <cusip>55291X109</cusip>
        <identifiers>
          <isin value="US55291X1090"/>
        </identifiers>
        <balance>30402493.58000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>30405533.83000000</valUSD>
        <pctVal>0.989892940682</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International Large Cap Value Fund</name>
        <lei>549300YEW2NKJCMQAR75</lei>
        <title>MFS International Large Cap Value Fund</title>
        <cusip>552743445</cusip>
        <identifiers>
          <isin value="US5527434451"/>
        </identifiers>
        <balance>8713149.02000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>160496205.02000000</valUSD>
        <pctVal>5.225169248596</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Research Fund</name>
        <lei>XFPOOG2XVG5TW1LZ8588</lei>
        <title>MFS Research Fund</title>
        <cusip>552981391</cusip>
        <identifiers>
          <isin value="US5529813918"/>
        </identifiers>
        <balance>5477573.91000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>306525035.78000000</valUSD>
        <pctVal>9.979333721211</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Value Fund</name>
        <lei>HV6C0RMURVKFUQ3UTB42</lei>
        <title>MFS Value Fund</title>
        <cusip>55273H353</cusip>
        <identifiers>
          <isin value="US55273H3536"/>
        </identifiers>
        <balance>7056975.71000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>374019712.42000000</valUSD>
        <pctVal>12.17671345850</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Commodity Strategy Fund</name>
        <lei>T0OEQ8I3F6DXUJCOV412</lei>
        <title>MFS Commodity Strategy Fund</title>
        <cusip>55276T826</cusip>
        <identifiers>
          <isin value="US55276T8264"/>
        </identifiers>
        <balance>39050410.39000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>158544666.16000000</valUSD>
        <pctVal>5.161634283159</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS Mid Cap Value Fund</name>
        <lei>BXHSCSBXYY1DOHP6E209</lei>
        <title>MFS Mid Cap Value Fund</title>
        <cusip>55273W475</cusip>
        <identifiers>
          <isin value="US55273W4758"/>
        </identifiers>
        <balance>9288427.64000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>313298664.43000000</valUSD>
        <pctVal>10.19985828824</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
      <invstOrSec>
        <name>MFS International Growth Fund</name>
        <lei>PTNAY8DC4VJYOKVJ5B83</lei>
        <title>MFS International Growth Fund</title>
        <cusip>552746356</cusip>
        <identifiers>
          <isin value="US5527463567"/>
        </identifiers>
        <balance>3135010.39000000</balance>
        <units>NS</units>
        <curCd>USD</curCd>
        <valUSD>159383928.02000000</valUSD>
        <pctVal>5.188957578821</pctVal>
        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
        <issuerCat>RF</issuerCat>
        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
          <isNonCashCollateral>N</isNonCashCollateral>
          <isLoanByFund>N</isLoanByFund>
        </securityLending>
      </invstOrSec>
    </invstOrSecs>
    <explntrNotes>
      <explntrNote note="Monthly returns presented in Item B.5(a) have been calculated without deducting any applicable sales loads or redemption fees." noteItem="B.5.a"/>
    </explntrNotes>
    <signature>
      <ncom:dateSigned>2026-03-27</ncom:dateSigned>
      <ncom:nameOfApplicant>MFS SERIES TRUST X</ncom:nameOfApplicant>
      <ncom:signature>James Byrne</ncom:signature>
      <ncom:signerName>James Byrne</ncom:signerName>
      <ncom:title>Assistant Treasurer</ncom:title>
    </signature>
  </formData>
  <documents>XXXX</documents>
</edgarSubmission>