Significance: This study meets a need for research that examines older adults’ motivation toward physical activity, as the majority of individuals over age 50 are underactive and not taking advantage of the health benefits of regular physical activity. This study also unites constructs from two continuum-type theoretical models, i.e. the transtheoretical model (TTM: Prochaska & DiClemente, 1983) and self-determination theory (SDT: Deci & Ryan, 1985). Although self-determination theory is increasingly being used as a theoretical foundation to explore motivation toward health behaviors, this model has yet to be applied to older adult physical activity behavior. SDT is particularly salient to older adults, given that experiences associated with aging may involve growing feelings of reduced personal autonomy and competence. The purpose of this study was to determine what intrinsic and extrinsic motives differentiate three activity levels (Inactives, Actives, Sustained Maintainers) in adults 50-79 years old, in order to gain greater understanding of the mechanisms of physical activity behavior change across stages-of-change in older adults. Design: 645 participants (M age: 63.8/sd: 8.3) completed self-administered questionnaires that determined exercise stage-of-change (Marcus et al., 1992) and assessed six motives associated with constructs from self-determination theory. These six motives were Enjoyment (Intrinsic motivation); Health & Fitness, Social/emotional benefits, Stress management (Self-determined extrinsic motivation); Weight Management, Appearance (Nonself-determined extrinsic motivation). The six measured motives are the factors yielded from principal components analysis of collected data from an administration of the Exercise Motivation Inventory-Revised (Markland & Ingledew, 1997) to this same population. It was hypothesized that intrinsic and self-determined extrinsic motivation (Deci & Ryan, 1985) would increase across the three activity levels. Statistical procedures included MANOVA, univariate post hoc tests, and discriminant function analyses. Gender and age categories (50-59, 60-69, 70-79) were considered as moderating factors. Results: MANOVA yielded main effects for activity level, gender, and age (p < .001), but no interaction effects among these three factors. Univariate post hoc paired-comparisons tests and discriminant function analyses between paired activity level groups indicated that intrinsic enjoyment of physical activity contributed to differentiating activity levels more than any extrinsic motives. Self-determined extrinsic motives of health and fitness, and social/emotional benefits also discriminated activity levels in certain population subgroups. Nonself-determined extrinsic motives of weight management and appearance did not discriminate activity levels. Findings give empirical support to the important role of self-determination, particularly intrinsic enjoyment, in the transition from inactive to regularly active lifestyles in older adults. Keyword(s): health promotion, older adult/aging issues, physical activity