<?xml version="1.0" encoding="UTF-8"?><edgarSubmission xmlns="http://www.sec.gov/edgar/ownership" xmlns:ns2="http://www.sec.gov/edgar/common">
    <headerData>
        <submissionType>144</submissionType>
        <filerInfo>
            <filer>
                <filerCredentials>
                    <cik>0001907682</cik>
                    <ccc>XXXXXXXX</ccc>
                </filerCredentials>
            </filer>
            <liveTestFlag>LIVE</liveTestFlag>



        </filerInfo>
    </headerData>
    <formData>
        <issuerInfo>
            <issuerCik>0001733294</issuerCik>
            <issuerName>GENERATION BIO CO.</issuerName>
            <secFileNumber>001-39319</secFileNumber>
            <issuerAddress>
                <ns2:street1>301 Binney Street</ns2:street1>
                <ns2:city>Cambridge</ns2:city>
                <ns2:stateOrCountry>MA</ns2:stateOrCountry>
                <ns2:zipCode>02142</ns2:zipCode>
            </issuerAddress>
            <issuerContactPhone>(617) 655-7500</issuerContactPhone>
            <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>ANTOINETTE D PAONE</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
            <relationshipsToIssuer>
                <relationshipToIssuer>Officer</relationshipToIssuer>
            </relationshipsToIssuer>
        </issuerInfo>
        <securitiesInformation>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <brokerOrMarketmakerDetails>
                <name>Morgan Stanley Smith Barney LLC</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>2914</noOfUnitsSold>
            <aggregateMarketValue>13443.74</aggregateMarketValue>
            <noOfUnitsOutstanding>59665758</noOfUnitsOutstanding>
            <approxSaleDate>04/25/2023</approxSaleDate>
            <securitiesExchangeName>NASD</securitiesExchangeName>
        </securitiesInformation>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>04/15/2023</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted Stock</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>2914</amountOfSecuritiesAcquired>
            <paymentDate>04/15/2023</paymentDate>
            <natureOfPayment>None</natureOfPayment>
        </securitiesToBeSold>
        <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>ANTOINETTE D PAONE</name>
                <address>
                    <ns2:street1>301 Binney Street</ns2:street1>
                    <ns2:city>Cambridge</ns2:city>
                    <ns2:stateOrCountry>MA</ns2:stateOrCountry>
                    <ns2:zipCode>02142</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <saleDate>01/26/2023</saleDate>
            <amountOfSecuritiesSold>2695</amountOfSecuritiesSold>
            <grossProceeds>14985.00</grossProceeds>
        </securitiesSoldInPast3Months>
        <noticeSignature>
            <noticeDate>04/25/2023</noticeDate>
            <planAdoptionDates>
                <planAdoptionDate>09/08/2022</planAdoptionDate>
            </planAdoptionDates>
            <signature>/s/ Antoinette Paone</signature>
        </noticeSignature>
    </formData>
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144
0001907682
XXXXXXXX
LIVE
0001733294
GENERATION BIO CO.
001-39319
301 Binney Street
Cambridge
MA
02142
(617) 655-7500
ANTOINETTE D PAONE
Officer
Common
Morgan Stanley Smith Barney LLC
1 New York Plaza
38th Floor
New York
NY
10004
2914
13443.74
59665758
04/25/2023
NASD
Common
04/15/2023
Restricted Stock
Issuer
N
2914
04/15/2023
None
N
ANTOINETTE D PAONE
301 Binney Street
Cambridge
MA
02142
COMMON
01/26/2023
2695
14985.00
04/25/2023
09/08/2022
/s/ Antoinette Paone