This study utilizes hierarchical regression techniques to examine the effect of extracurricular participation on social capital and academic achievement. Extracurricular participation in specific activities is revealed to operate as a means for obtaining membership in academically oriented peer groups. The adoption of pro-academic norms as a result of socialization within these groups is presented as one mechanism through which participation can positively affect achievement. Membership in an academically oriented peer group is revealed to act as a significant mediator of participation effects and demonstrates a positive effect on mathematics achievement among male students. Consequently, the academic disposition of peers and friends in these groups constitutes a form of social capital as defined by Coleman and underscores the necessity of examining homophilial, as well as heterophilial relationships as potential sources of social capital.