The purpose of these studies was to examine the relationship between perceptions of exercise-related changes (i.e., perceived mastery and physical change) and certainty with regard to the self-as-exerciser. It was hypothesized that seeing “change” would be associated with more favorable levels of exercise self-certainty and behavior relative to “no change.” Online surveys were repeatedly administered across 4 months (Study 1) and 4 weeks (Study 2) to 196 university students (M_age = 20.17), and 250 community dwellers (M_age = 38.44), respectively. Data were analyzed via latent variable modeling procedures. Consistent with hypotheses, latent classes (i.e., subgroups) reflecting interindividual differences in levels and trajectories of perceived change were associated with distinct patterns of selfcertainty and exercise behavior. The findings suggest that adults who experience mastery of skills and physiological changes also have greater self-certainty and exercise more regularly than those who do not see progress or feel as certain of their exercise identity.