Data from a 1991 survey of five Nigerian towns are used to examine currently married men's and women's perceptions of AIDS which, together with other socioeconomic factors, are then related to extramarital sexual behaviour. An overwhelming majority of the respondents have accurate information about AIDS. In particular, most associate HIV/AIDS transmission with multiple sexual partners, though only one-third of them think that the fear of AIDS has limited casual sex in their communities. About 54 per cent of men and 39 per cent of women have had extramarital relations, with 18 per cent of men and 11 per cent of women having done so in the previous week. The incidence of extramarital relations varies considerably by respondents' level of education, type of marriage, religion, and spousal closeness. More importantly, knowledge of multiple sexual partners as a risk factor for HIV/AIDS is inversely related to extramarital affairs. The study underscores the link between knowledge and behaviour, and calls for a well-articulated campaign designed to educate the populace about the threat of AIDS, with the aim of modifying both premarital and extramarital sexual behaviour, thereby reducing the risk factor for HIV through heterosexual relations which is the main mode of transmission in Nigeria.