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      <regFileNumber>811-04492</regFileNumber>
      <regCik>0000783740</regCik>
      <regLei>5493001MUVNRNUPJY164</regLei>
      <regStreet1>111 HUNTINGTON AVENUE</regStreet1>
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      <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>2020-05-31</repPdEnd>
      <repPdDate>2020-05-31</repPdDate>
      <isFinalFiling>N</isFinalFiling>
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      <totLiabs>2655010.06</totLiabs>
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      <assetsInvested>0.00000000</assetsInvested>
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        <name>MFS Growth Fund</name>
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        <name>MFS New Discovery Value Fund</name>
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        <fairValLevel>1</fairValLevel>
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        <name>MFS New Discovery Fund</name>
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        <name>MFS Research Fund</name>
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        <name>MFS Global Real Estate Fund</name>
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        <fairValLevel>1</fairValLevel>
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        <name>MFS International Value Fund</name>
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        <fairValLevel>1</fairValLevel>
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      </invstOrSec>
      <invstOrSec>
        <name>MFS Emerging Markets Equity Fund</name>
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        <fairValLevel>1</fairValLevel>
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      <invstOrSec>
        <name>MFS Value Fund</name>
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        <fairValLevel>1</fairValLevel>
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          <isLoanByFund>N</isLoanByFund>
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      </invstOrSec>
      <invstOrSec>
        <name>MFS Institutional Money Market Portfolio</name>
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        <fairValLevel>1</fairValLevel>
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      <invstOrSec>
        <name>MFS International Growth Fund</name>
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        <fairValLevel>1</fairValLevel>
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          <isCashCollateral>N</isCashCollateral>
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      </invstOrSec>
      <invstOrSec>
        <name>MFS Mid Cap Growth Fund</name>
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        <balance>8342750.34100000</balance>
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        <curCd>USD</curCd>
        <valUSD>196054633.01000000</valUSD>
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        <payoffProfile>Long</payoffProfile>
        <assetCat>EC</assetCat>
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        <invCountry>US</invCountry>
        <isRestrictedSec>N</isRestrictedSec>

        <fairValLevel>1</fairValLevel>
        <securityLending>
          <isCashCollateral>N</isCashCollateral>
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      </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>2020-06-12</ncom:dateSigned>
      <ncom:nameOfApplicant>MFS SERIES TRUST X</ncom:nameOfApplicant>
      <ncom:signature>Kasey Phillips</ncom:signature>
      <ncom:signerName>Kasey Phillips</ncom:signerName>
      <ncom:title>Assistant Treasurer</ncom:title>
    </signature>
  </formData>
</edgarSubmission>
