CONTEXT Young people in South Africa are at high risk of HIV infection. Because first sexual experiences may influence a young person's HIV risk, a better understanding of coital debut is needed. METHODS Data from a nationally representative survey that included 7,692 sexually active South African youth aged 15-24 were used to assess characteristics related to sexual debut and to respondents' first sexual partner. Poisson regression analyses were conducted to identify relationships among these characteristics and partner age differences, early coital debut (i.e., before age 15), forced sex with one's first partner and nonuse of condoms at first sex. RESULTS Eighteen percent of young men and 8% of young women reported early coital debut. The likelihood of early debut was elevated among females and males who had had an older first partner (adjusted prevalence ratio, 1.1 per year) and among females who had had forced sex (2.5). Lack of condom use at first sex was associated with early coital debut (1.5) and forced sex (1.6) for males. Among females, the likelihood of nonuse was elevated for respondents who had had an early debut but had not had forced sex (1.3), and among those who had had both a later debut and forced sex (1.4). CONCLUSIONS Early coital debut is associated with factors that may increase a young person's risk for HIV infection, such as forced sex and having older partners. Intervention efforts should encourage youth to delay coital debut and promote strategies to make young people's first sexual experience safer.