Recent evidence suggests that the burden of new HIV infections in sub-Saharan Africa is concentrated among young people, especially females. Even in a country such as South Africa where knowledge among young people of how to protect oneself from infection is rather high, such information may not always be usable in daily situations of economic and social disadvantage that characterise many of their lives. Despite this possibility, there are surprisingly few definitive studies that examine the effects of socioeconomic status on HIV risk and prevention behaviours among youth in South Africa. Using household survey data collected in 2001, this study investigates how socioeconomic disadvantage has influenced the sexual behaviour and experiences of 4 000 young women and men aged 14–24 years in KwaZulu-Natal province — an area characterised by high HIV prevalence and high rates of poverty and inequality. Socioeconomic disadvantage, measured here as low relative household wealth, is found to be associated with a variety of unsafe sexual behaviours and experiences, particularly for females. Among young women low wealth is associated with earlier sexual debut, having had multiple sexual partners in the year before the survey, and lower chances of condom use at last sex. It is also associated with increased chances that first female sexual experience is non-consensual and with higher odds of females having traded sex and having experienced physically forced sex. For females and males, low wealth reduces the chances of discussing safe-sex practices with the most recent sexual partner. Without sufficient attention, during the design and placement of HIV prevention programmes, to the gendered economic and social conditions in which individuals live — conditions that can make people more or less vulnerable to behaviours and experiences that may lead to infection — the potential effectiveness of the global response to HIV/AIDS is sacrificed.