Peer group contextual effects of aggressive behavior among middle school students (6th-8th graders) were examined using a short-term longitudinal design. More specifically, the homophily hypothesis that peer group membership influences individual-level bullying and fighting was evaluated with multilevel sex-specific models of individual- and peer-level aggression scores. Peer groups were identified via social network analysis. Intraclass correlation coefficients yielded through hierarchical linear modeling demonstrated substantial within-group similarity on self-reported bullying and fighting, suggesting that students affiliate with individuals who bully and fight at the same frequency. Peer group bullying and fighting was associated with individual-level behavior, even after controlling individual baseline levels for males and females. However, peer contextual effects explained more variance in individual bullying than individual fighting. This differential impact of peer group membership suggests that future studies consider peer relations across subtypes of aggression.