This study examined at-risk boys' social self-efficacy and physical activity self-efficacy within Bandura's self-efficacy framework. A total of 97 boys, aged between 10 and 13 years, attending a summer sports camp completed questionnaires assessing their social self-efficacy, physical activity self-efficacy, prosocial behaviors, and effort. Results indicated that social self-efficacy and physical activity self-efficacy were clearly distinguishable. However, the two constructs had a strong positive correlation. Both social self-efficacy and physical activity self-efficacy predicted prosocial behaviors significantly, with social self-efficacy having a stronger predictive power. Physical activity self-efficacy was a better predictor of effort than social self-efficacy. This study provides initial empirical evidence supporting Bandura's conceptualization of the domain-specific features and predictive power of self-efficacy in a summer sports camp setting, and thus enables a better understanding of the nature and effects of self-efficacy.