This study investigated to what extent team membership predicts on- and off-field antisocial and prosocial behavior in (pre)adolescent athletes. Effects of team-membership were related to characteristics of the team environment, such as relational support from the coach towards team members, fair play attitude and sociomoral reasoning within the team, and sociomoral climate. The sample consisted of N=331 male soccer players. Multilevel analyses revealed that 21% of the variance in off-field antisocial behavior, and 8% and 14% of the variance in on-field antisocial and prosocial behavior, respectively, could be attributed to characteristics of the sporting environment, including relational support from the coach, exposure to high levels of sociomoral reasoning about sports dilemmas, and positive team attitude toward fair play. The results highlight the importance of contextual factors in explaining both antisocial and prosocial behavior in adolescent athletes and emphasize the role of organized youth sports as a socialization context. © 2007 The Association for Professionals in Services for Adolescents.