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Background: There have been very limited prospective studies examining social-cognitive models within stages of behavior change in the exercise domain.Purpose: We examined the utility of the theory of planned behavior (TPB), incorporating self-identity and descriptive norm constructs, to predict exercise behavior across the stages of change, in individuals with type 2 diabetes.Methods: Data were obtained from a longitudinal study. Multi-group structural equation modeling was used to estimate the association between extended TPB constructs and exercise within different stages groups.Results: 647 individuals completed a self-report questionnaire at baseline and at 3 months follow-up. The extended TPB model explained 8–15% variance of exercise behavior and 42–81% variance of exercise intention within three stages groups in the cross-sectional design. The extended TPB model explained 4%-13% variance of exercise behavior and 42–66% variance of exercise intention in the longitudinal design. Intention was significantly related to exercise behavior in the pre-action and action stages. Self-identity, perceived behavioral control and descriptive norms were stronger predictors of intention in different stages.Conclusion: Discontinuity patterns in the extended theory of planned behavior for the different stages groups were found. Intention was a significant predictor of exercise in the pre-action and action stages at 3 months.