The current study explored how prosocial behavior may moderate how aggression is related to the features of adolescents' friendships. Young adolescents (N = 910) completed friendship nominations in the fall and spring of their first year of middle school. Behavioral nominations of aggression and prosocial behavior were also collected in the fall. A subsample (N = 374) of adolescents and their reciprocated friends reported on friendship quality. Prosocial behavior moderated how aggression was related to the likelihood of having a mutual best friendship in the fall. Dyadic data analyses also revealed that when prosocial behavior was low, aggression was negatively related to friendship quality. Examination of temporal patterns in best friendships indicated that when prosocial behavior was low, aggression was marginally predictive of having different best friends in the fall and spring relative to having a stable best friendship across the school year.