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    <headerData>
        <submissionType>144</submissionType>
        <filerInfo>
            <filer>
                <filerCredentials>
                    <cik>0001603347</cik>
                    <ccc>XXXXXXXX</ccc>
                </filerCredentials>
            </filer>
            <liveTestFlag>LIVE</liveTestFlag>



        </filerInfo>
    </headerData>
    <formData>
        <issuerInfo>
            <issuerCik>0001515673</issuerCik>
            <issuerName>ULTRAGENYX PHARMACEUTICAL INC.</issuerName>
            <secFileNumber>001-36276</secFileNumber>
            <issuerAddress>
                <ns2:street1>60 Leveroni Court</ns2:street1>
                <ns2:city>Novato</ns2:city>
                <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                <ns2:zipCode>94949</ns2:zipCode>
            </issuerAddress>
            <issuerContactPhone>(415) 483-8800</issuerContactPhone>
            <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>THEODORE ALAN HUIZENGA</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
            <relationshipsToIssuer>
                <relationshipToIssuer>Officer</relationshipToIssuer>
            </relationshipsToIssuer>
        </issuerInfo>
        <securitiesInformation>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <brokerOrMarketmakerDetails>
                <name>Morgan Stanley Smith Barney LLC Executive Financial Services</name>
                <address>
                    <ns2:street1>1 New York Plaza</ns2:street1>
                    <ns2:street2>38th Floor</ns2:street2>
                    <ns2:city>New York</ns2:city>
                    <ns2:stateOrCountry>NY</ns2:stateOrCountry>
                    <ns2:zipCode>10004</ns2:zipCode>
                </address>
            </brokerOrMarketmakerDetails>
            <noOfUnitsSold>2163</noOfUnitsSold>
            <aggregateMarketValue>88249.42</aggregateMarketValue>
            <noOfUnitsOutstanding>71488598</noOfUnitsOutstanding>
            <approxSaleDate>09/07/2023</approxSaleDate>
            <securitiesExchangeName>NASDAQ</securitiesExchangeName>
        </securitiesInformation>
        <securitiesToBeSold>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <acquiredDate>09/01/2023</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted Stock</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>ISSUER</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>163</amountOfSecuritiesAcquired>
            <paymentDate>09/01/2023</paymentDate>
            <natureOfPayment>Not Applicable</natureOfPayment>
        </securitiesToBeSold>
        <securitiesToBeSold>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <acquiredDate>09/07/2023</acquiredDate>
            <natureOfAcquisitionTransaction>Stock Option Exercise</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>ISSUER</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>2000</amountOfSecuritiesAcquired>
            <paymentDate>09/07/2023</paymentDate>
            <natureOfPayment>Cash</natureOfPayment>
        </securitiesToBeSold>
        <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>THEODORE ALAN HUIZENGA</name>
                <address>
                    <ns2:street1>60 Leveroni Court</ns2:street1>
                    <ns2:city>Novato</ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94949</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <saleDate>09/06/2023</saleDate>
            <amountOfSecuritiesSold>87</amountOfSecuritiesSold>
            <grossProceeds>3326.00</grossProceeds>
        </securitiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>THEODORE ALAN HUIZENGA</name>
                <address>
                    <ns2:street1>60 Leveroni Court</ns2:street1>
                    <ns2:city>Novato</ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94949</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <saleDate>09/01/2023</saleDate>
            <amountOfSecuritiesSold>883</amountOfSecuritiesSold>
            <grossProceeds>32706.00</grossProceeds>
        </securitiesSoldInPast3Months>
        <noticeSignature>
            <noticeDate>09/07/2023</noticeDate>
            <planAdoptionDates>
                <planAdoptionDate>02/24/2023</planAdoptionDate>
            </planAdoptionDates>
            <signature>/s/ Theodore Alan Huizenga</signature>
        </noticeSignature>
    </formData>
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144
0001603347
XXXXXXXX
LIVE
0001515673
ULTRAGENYX PHARMACEUTICAL INC.
001-36276
60 Leveroni Court
Novato
CA
94949
(415) 483-8800
THEODORE ALAN HUIZENGA
Officer
Common
Morgan Stanley Smith Barney LLC Executive Financial Services
1 New York Plaza
38th Floor
New York
NY
10004
2163
88249.42
71488598
09/07/2023
NASDAQ
COMMON
09/01/2023
Restricted Stock
ISSUER
N
163
09/01/2023
Not Applicable
COMMON
09/07/2023
Stock Option Exercise
ISSUER
N
2000
09/07/2023
Cash
N
THEODORE ALAN HUIZENGA
60 Leveroni Court
Novato
CA
94949
Common
09/06/2023
87
3326.00
THEODORE ALAN HUIZENGA
60 Leveroni Court
Novato
CA
94949
Common
09/01/2023
883
32706.00
09/07/2023
02/24/2023
/s/ Theodore Alan Huizenga