This study seeks to examine the reasons why young women aged 14-20 in Maputo, Mozambique were engaging in risky sexual behaviour, and to ascertain whether the spread of HIV is due to socioeconomic factors and/or patriarchal beliefs and mores, or both. It is based on in-depth interviews and questionnaires among 182 girls in two secondary schools in Maputo, one attended primarily by working-class and one primarily by middle-class students. The findings suggest that while gender dynamics work against women overall, middle-class young women had fewer sexual partners, used condoms more often, seemed willing to challenge gender norms and were more assertive than their working class counterparts, which placed them at potentially more of an advantage in sexual negotiation. Working-class young women, for whom the interlocking categories of gender and class operated, were more accepting of gender power differentials, were less assertive and tended to be dependent on their partners for material needs more often, which served to weaken their bargaining power in relation to safe sexual behaviour and rendered them more vulnerable.