Studies suggest that Mexican–American youths who feel attached to their ethnic group engage in more prosocial behaviors. However, the psychological mechanisms that account for this association are not clear. Drawing on social identity and self-categorization theories, we examined whether the association between ethnic group attachment and tendencies to engage in six distinct types of prosocial behaviors was mediated by familism and Mainstream American values among Mexican American youths. Ethnic group attachment, familism, Mainstream American values, and prosocial tendencies were assessed among 207 Mexican–American early adolescents using an interview format. Latent variable path models showed that ethnic group attachment was associated with greater tendencies to engage in compliant, emotional, dire, and anonymous helping, and that each of these associations was at least partially mediated by greater familism values. Mainstream American values were related to greater tendencies to engage in public prosocial behavior and less altruistic behavior, but did not mediate the associations between ethnic group attachment and prosocial tendencies. The theoretical and practical implications of these findings are addressed. Copyright © 2010 John Wiley & Sons, Ltd.