Background. Low levels of physical activity (PA) and highly sedentary leisure habits (SLH) in youth may establish behavioral patterns that will predispose youth to increased chronic disease risk in adulthood. The purpose of this paper was to examine associations of demographic and psychosocial factors with self-reported PA and SLH in young adolescents. Methods. A general linear mixed model predicted self-reported PA and SLH in the spring from demographic and psychosocial variables measured the previous fall in 3798 seventh grade students. Results. PA and SLH differed by race, with Caucasian students reporting among the highest PA and lowest SLH. Perceptions of higher academic rank or expectations predicted higher PA and lower SLH. Depressive symptomatology predicted higher SLH scores but not PA. Higher self-reported value of health, appearance, and achievement predicted higher PA and lower SLH in girls. Girls who reported that their mothers had an authoritative parenting style also reported higher PA and lower SLH. Conclusions. Determinants of PA and SLH appear to differ from each other, particularly in boys. Development of effective programs to increase PA and/or decrease SLH in young adolescents should be based on a clear understanding of the determinants of these behaviors.