Little is known about the influences of peers on the sexual activity of adolescents in sub-Saharan Africa. Better understanding of these issues could lead to more effective sexual and reproductive health interventions. Using two waves of survey data from 1,275 adolescents in two southeastern Ghanaian towns, we examine age, sex, and community differences in peer group characteristics. We also examine prospective associations between peer group characteristics and self-reported sexual initiation and multiple partnerships during a 20-month follow-up period. Sex differences in peer-context variables were small. Affiliation with antisocial peers and perceived peer norms favoring sex increased the odds of transition to first sex. Having more friends increased the odds among younger respondents of acquiring multiple new sexual partners. Among males, perceived peer norms favoring sex increased the odds of acquiring multiple partners. We discuss the implications of these findings for adolescent sexual and reproductive health intervention strategies in sub-Saharan Africa, and conclude that peer-based interventions may be best suited to the needs of at-risk adolescent boys.