Physical activity and fitness are important factors in the primary prevention of chronic disease in youth, yet physical activity levels actually diminish during adolescence. Furthermore, adolescence is an important period for learning health-related behaviors that will carry into adulthood. A better understanding of psychosocial and environmental determinants of physical activity for adolescents will assist efforts in school and community settings to promote lifelong physical activity. Researchers have highlighted the need for further examination of these modifiable determinants of physical activity in adolescents. Therefore, the purpose of the study was to develop a questionnaire to assess psychosocial and environmental determinants of physical activity in adolescents. Six social cognitive theory constructs were used to frame the questionnaire: (a) self-efficacy, (b) behavioral capability, (c) outcome expectations, (d) observational learning, (e) reinforcement, and (f) environment. A total of 46 items were matched to the theoretical constructs and evaluated using expert and content validation procedures. The questionnaire was administered to 215 (105 male, 110 female) adolescents from health and physical education classes in three urban schools. The majority (82%) of participants were African-American. A confirmatory factor analysis was performed, and the analysis did not indicate a good model fit. The Root Mean Square Error of Approximation was adequate (.088), however, the fit indices were low. Therefore, an exploratory factor analysis (EFA) was performed using maximum likelihood analysis with varimax rotation. The EFA produced 13 factors with eigenvalues greater than 1, accounting for 64% of the variance. Eleven of the 13 factors were eliminated because of low loadings (l < .40) or insufficient number of items. The resulting 2-factor structure comprised 21 items representing personal (16) and social (5) factors. The internal consistency of the two factors was .89 and .74, respectively. The researchers plan to conduct additional confirmatory and validation analyses to revise the questionnaire. The development of a valid and reliable questionnaire will contribute to the assessment of physical activity determinants, and will help in the development of strategies to increase physical activity among adolescents. The questionnaire may be applied to: (a) predict physical activity related behaviors among adolescents, (b) assess the effectiveness of interventions on adolescents’ perceptions of physical activity determinants, and (c) explain adolescents’ movement through the transtheoretical model and other theories related to physical activity.