The current study explored the role of school-based friendship networks in adolescents' engagement in physical activity (PA). It was hypothesized that similar participation in PA would be a basis for friendship formation, and that friends would also influence behavior. Whether these processes were mediated through cognitive mechanisms was also explored. Self-reported participation in PA, cognitions about PA, and friendship ties to grade-mates were measured in two cohorts of Australian grade eight students (N = 378; M age = 13.7) three times over the 2008 school year. Interdependence between the friendship networks and PA was tested using stochastic actor-based models for social networks and behavior. The results showed that participants tended to befriend peers who did similar amounts of PA, and subsequently emulated their friends' behaviors. Friends' influence on PA was not found to be mediated through adolescents' cognitions about PA. These findings show that there is a mutually dependent relationship between adolescent friendship networks and PA; they highlight how novel network-based strategies may be effective in supporting young people to be physically active.