Male college students (N=395) completed anonymous surveys to report personal attitudes supporting sexual aggression and estimated the attitudes of their peers. Participants also indicated their willingness to intervene against a peer if they witnessed sexual aggression. Although both personal and peer attitudes were correlated with willingness to intervene, in regression analyses only perceived peer attitudes emerged as a significant predictor of willingness to intervene. Results suggest that personal attitudes supporting sexual aggression are not as relevant to men's willingness to intervene against sexual aggression as are perceived peer norms regarding sexual aggression. Findings are relevant to sexual assault prevention education with men, suggesting that attempts to encourage bystander intervention may be best presented in the context of challenging perceived norms. (Published Abstract)