Objective: To determine whether individual factors influenced rates of physical activity change in response to a school-based intervention. Methods: Sedentary adolescent females (N = 63) participated in a 9-month physical activity program. Weekly levels of leisure-time physical activity were reported using an interactive website. Results: Change in vigorous activity was more positive among participants who had higher fitness and lower friend support at baseline. Change in moderate activity was more positive among participants who had lower fitness and external barriers, and higher internal barriers at baseline. Conclusions: Adolescent females responded differentially to a physical activity intervention depending on individual characteristics.