Background: Physical activity (PA) declines sharply and rapidly during adolescence, especially among girls, posing a risk for inactivity and obesity in adulthood. This study identified personal, behavioral, and socioen-vironmental correlates of concurrent and 6-month longitudinal PA among adolescent girls. Methods: Data were gathered from 356 adolescent girls (mean age 15.8 ± 1.2 years; > 75% racial/ethnic minorities) in the Minneapolis/St. Paul area in 2007-2009. Linear regression analyses controlling for age, race/ethnicity, and school were conducted predicting baseline and follow-up levels of total PA and moderate-to-vigorous PA (MVPA) assessed via 3-Day Physical Activity Recall. Models were fit for each correlate individually and for all correlates together, mutually adjusted. Results: For concurrent PA, significant positive predictors when adjusting for the influence of all other variables included self-efficacy, support from friends and teachers, and friends' PA. Total screen time and distance from school to PA resources related inversely to concurrent PA. In mutually-adjusted models, 6-month PA was positively related to self-worth, family support, and parent PA and inversely related to total screen time. Conclusions: PA interventions with adolescent girls might be enhanced by involving adolescents' social networks and also by helping adolescents feel better about their self-worth and athletic abilities.