INTRODUCTION:HIV pre-exposure prophylaxis (PrEP) has been increasingly adopted by gay and bisexual men (GBM). Little is known about whether individual GBM change their sexual behavior after PrEP initiation. METHODS:Following Lives Undergoing Change (Flux) is a national, online, prospective observational study among Australian GBM. Using McNemar statistics, we compare rates of sexual behaviors before and coincident with PrEP initiation among 1518 non-HIV-positive men recruited between August 2014 and July 2017 who had not commenced PrEP at baseline and who completed at least one 6-monthly follow-up surveys by July 2018. RESULTS:The proportion of men using PrEP rose to 24.2% over time. In total, 348 men initiated PrEP during follow-up. PrEP initiators were more likely to report particular sexual behaviors during the follow-up period that they commenced PrEP compared with the period immediately prior: receptive condomless anal intercourse with casual partners increased from 31.0% to 48.9% (McNemar < 0.001); mean partner number increased from 21.96 partners to 34.55 partners (p-trend < 0.001). Among the 1170 men who did not initiate PrEP, prevalence of these behaviors remained lower and stable. Sexual sensation-seeking and gay social engagement were both higher among men who commenced PrEP. CONCLUSIONS:GBM tended to increase their engagement in "adventurous" sexual behaviors after PrEP initiation. Sexual behaviors among men who did not initiate PrEP were less common and did not change over time.