This study examines Self-Determination Theory (SDT) in a physical activity context, using a prospective design to predict leisure time physical activity. We expected need satisfaction and self-determined motivation to predict physical activity 1 month later. One hundred sixteen undergraduate students completed two questionnaires, 1 month after the other. As anticipated, a path analysis revealed the proposed model to fit the data. Specifically, each psychological need positively predicted self-determined motivation, and competence negatively predicted nonself-determined motivation. Self-determined motivation was then found to predict physical activity 1 month later, while nonself-determined motivation was not a significant predictor. These findings support the theoretical model proposed by SDT and the implications of these findings are discussed.