Epidemiologists have suggested that racial disparities in sexually transmitted infections are due to more concurrency in Black sexual networks. We argue that a relative shortage of Black males, caused by higher Black incarceration, increases concurrency. We test this hypothesis using data from the 2000 Census and National Longitudinal Survey of Youth (1997). Because incarceration does not affect sex ratios until early adulthood, we focus on males from adolescent to adulthood, and use variation in an age-, race-, and state-level sex ratio to identify its effect on risky sexual behavior. Our evidence suggests that changes in the sex ratio causes Blacks to have between 1.2 - 2.0 additional partners a year, whereas its effect on condom-use is unclear.