<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-04-19</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001092896</issuerCik>
        <issuerName>MFS CALIFORNIA MUNICIPAL FUND</issuerName>
        <issuerTradingSymbol>CCA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001223434</rptOwnerCik>
            <rptOwnerName>CONNORS THOMAS H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>MFS INVESTMENT MANAGEMENT</rptOwnerStreet1>
            <rptOwnerStreet2>111 HUNTINGTON AVENUE</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>ASSISTANT SECRETARY</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <ownerSignature>
        <signatureName>CHRISTOPHER R. BOHANE</signatureName>
        <signatureDate>2022-10-25</signatureDate>
    </ownerSignature>
</ownershipDocument>