Gender differences in sexual behavior as a consequence of migration have been ignored in both the migration and the HIV literature in China. This study examines differences among temporary migrants in terms of sexual behavior and factors that make female migrants more vulnerable to the risk of acquiring HIV infection. Results suggest that the interplay of migration and gender renders female temporary migrants particularly vulnerable to engaging in casual and commercial sex. Although male temporary migrants do not differ from male nonmigrants in prevalence of casual and commercial sex, the prevalence rates of casual and commercial sex for female temporary migrants are found to be 14 and 80 times those for female nonmigrants, respectively. Female temporary migrants' higher unemployment rate and concentration in the service and entertainment sectors are keys to understanding differences in the prevalence of casual and commercial sex among temporary migrants according to sex. Policy measures to promote female temporary migrants' equal access to employment are urgently needed to improve their economic well-being and to reduce their risky sexual behavior.