An extended theory of planned behavior (Ajzen, 1991), incorporating the post-decisional phase of behavior and constructs from self-determination theory (Deci & Ryan, 1985), was tested for physical activity using a prospective survey design. Participants (N = 172) completed measures of intentions, attitudes, subjective norms, perceived behavioral control (PBC), self-determined motivation, continuation intentions, and chronically accessible physical activity motives. Participants completed a self-report measure of physical activity 3 weeks later. Path analysis supported the predictive utility of the proposed model. Importantly, the effect of continuation intentions of success on physical activity behavior was moderated by chronically accessible physical activity motives. Findings underscore the importance of taking into account continuation intentions, self-determined motivation, and individuals' chronically accessible motives when developing physical-activity-promoting interventions.