Examined the occurrence of selective peer affiliation, and its impact on the development of aggressive behavior in four classrooms serving 72 preschool-age, high-risk boys and girls. Children classified as aggressive and nonaggressive were both highly selective in their peer affiliations, spending the majority of their time with a few same-sex classmates. Children generally established strong, stable, mutual affiliations with peers similar to themselves in aggression, but aggressive children had more difficulty establishing such affiliations. The interaction of peer dyads containing at least one aggressive child were characterized by more frequent, lengthy, and intense conflicts regardless of the affiliative relationship characterizing the dyad. The amount of time children spent interacting with aggressive peers predicted changes in observed and teacher-rated aggressiveness 3 months later.