In this study we examined adolescents' physical activity (PA) stability as well as individual differences in PA across grades 7-9. In addition, we tested the predictive role of adolescents' self-determination and goal orientations in physical education at grade 6 in relation to their PA development. Adolescents' (N = 812) self-reported PA was measured five times and physical education motivation once. Results revealed a decreasing trajectory of PA, with individual differences diminishing over time. Although boys' showed a consistently higher PA level, gender did not moderate the change in PA. Finally, self-determination and task orientation (boys only) predicted PA levels but did not predict change in PA. (Contains 2 tables, 2 figures, and 1 note.)