This research examined the independent and interactional contributions of peer experiences and group aggression to youth behavioral adjustment in short-term residential treatment. Participants were 219 youth (M age = 12.70, SD = 2.76; 71 % male) nested in 28 same-age, same-sex treatment groups. Sociometric interviews assessed social preference and victimization. Daily behavioral observations by staff assessed overall levels of treatment group aggression, as well as aggressive, withdrawn, and prosocial responses to specific social events. End-of-summer behavioral responses (to all events; to peers; to adults) were predicted, controlling for initial levels of these responses. Social preference predicted higher end-of-summer prosocial responses, and victimization predicted lower prosocial and higher withdrawn responses. Each interacted with group aggression in some analyses, with more positive peer experiences only predicting more favorable responses in groups that were low or average in aggression. Interactant-specific analyses revealed that some of these associations were broad, whereas others applied only to adults. For example, group aggression moderated the association between social preference and aggressive responses to adults but not peers. Gender differences were also interactant-specific. Results highlight the importance of peer experiences in group treatment and underscore the value of both aggregation and disaggregation over interactants in analyses of behavioral adjustment.