The purpose of this investigation was to develop and refine a psychometric measure of self-motivation and assess its relationship to adherence to programs of habitual physical activity. Following construction of a Self-Motivation Inventory, a series of studies were conducted involving the pretesting of undergraduate males and females (N= 401) and subsequent validation work involving intercollegiate women athletes (N= 64) and adult males (N= 66) in actual exercise settings. Results of the psychometric work provided evidence for the logical validity, internal consistency, and test-retest reliability of the self-motivation construct. In addition, convergent and discriminant evidence was provided for construct validity through self-motivation's pattern of association with other conceptually relevant psychometric variables. Finally, from the standpoint of behavioral validation, self-motivation proved to be the best discriminator between exercise adherers and dropouts among the psychological variables employed and was strongly related to program adherence in both exercise settings. In fact, when combined with selected morphologic variables in a psychobiologic framework, self-motivation scores were found to accurately classify participants according to their adherence status in approximately 80% of all cases and to account for nearly 50% of the variance in exercise adherence behavior.