The purpose of this study was to describe the relationships between Social Cognitive Theory (SCT) variables and frequency of moderate-vigorous physical activity in a sample of high school students. Days of moderate and vigorous physical activity were assessed using a previously validated one-day recall instrument administered on seven consecutive days. The SCT variables tested were: self-regulation, social situation, social outcome expectations, physical appearance outcome expectations, general health outcome expectations, negative outcome expectations, self-efficacy for ability, and self-efficacy for barriers. The SCT variables studied are potentially modifiable via educational interventions. Results revealed that 15 % of the subjects reported 3 or more days of physical activity. Four of the SCT were retained in the regression model accounting for 31 % of the variance in days or moderate-vigorous physical activity. SCT is useful for understanding frequency of moderate-vigorous physical activity among adolescents.