Given the dramatic increase in depression that occurs during early adolescence in girls, interventions must address the needs of girls. The authors examined whether a depression prevention program, the Penn Resiliency Program, was more effective for girls in all-girls groups than in co-ed groups. Within co-ed groups, the authors also tested whether there were greater effects for boys than for girls. Participants were 208 11- to 14-year-olds. Girls were randomly assigned to all-girls groups, co-ed groups, or control. Boys were assigned to co-ed groups or control. Students completed questionnaires on depressive symptoms, hopelessness, and explanatory style before and after the intervention. Girls groups were better than co-ed groups in reducing girls' hopelessness and for session attendance rates but were similar to co-ed groups in reducing depressive symptoms. Co-ed groups decreased depressive symptoms, but this did not differ by gender. Findings support prevention programs and suggest additional benefits of girls groups. (Contains 1 table.)