Participants in the study were recreational runners who completed measures of their orientation to exercise, the Five Factor Model of personality, self-efficacy as a specific adaptation (a socio-cognitive construct of personality) and measures of subjective well-being (life satisfaction) and eudaimonic well-being (life engagement). Consistent with previous research, task-oriented (internally focused) motivation to exercise was positively related to extraversion and to conscientiousness, and ego-oriented (externally focused) motivation was positively related to extraversion. Also consistent with previous research, self-efficacy and measures of well-being were positively related to extraversion and conscientiousness. Mediational analyses found that well-being mediated relationships between task-oriented motives and both extraversion and conscientiousness. Self-efficacy mediated the relationship between ego-oriented motives and extraversion. The implications of these results for the study individual differences in exercise motivation are discussed.