This paper compares the sexual behaviors of young people in South Africa (SA) and the United States (US) with the aim of better understanding the potential role of sexual behavior in HIV transmission in these two countries that have strikingly different HIV epidemics. Nationally representative, population-based surveys of young people ages 18-24 years from SA (n = 7,548) and the US (n = 13,451) were used. The prevalence of HIV was 10.2% in SA and 1% in the US. Young women and men in the US reported an earlier age of first sex than those in SA (mean age of coital debut for women: US [16.5], SA [17.4]; for men: US [16.4], SA [16.7]). The median number of lifetime partners is higher in the US than in SA: women: US (4), SA (2); men: US (4), SA (3). The use of condom at last sex is reported to be lower in the US than in SA: women: US (36.1%), SA (45.4%); men: US (48%), SA (58%). On average, young women in SA report greater age differences with their sex partners than young women in the US. The authors conclude that young people in the US report riskier sexual behaviors than young people in SA, despite the much higher prevalence of HIV infection in SA. Factors above and beyond sexual behavior likely play a key role in the ongoing transmission of HIV in South African youth, and thus should be urgently uncovered to develop maximally effective prevention strategies.