The present study examined undergraduate students' physical activity during transition from high school to first-year university. Students' (N = 127) self-efficacy for coping with barriers to physical activity was investigated both as a predictor of physical activity and mediator of the relationship between pretransition and first-year physical activity. Physical activity was found to track moderately from pretransition to first year (r [subscript s] = 0.58). Self-efficacy for coping with barriers predicted physical activity (R [superscript 2] [subscript adjusted] = 0.18, p less than 0.05) and partially mediated the relationship between pretransition and first-year physical activity. Results support an interpretation that future intervention efforts could target first-year students' self-efficacy for coping with barriers to help curb the decline in physical activity that occurs during the transition to university life. (Contains 2 tables.)