This study examined the relations among activity participation, expressive identity, and wellness in a sample of 115 high school students. Results indicated that most adolescents identify several activities as self-defining. Within-person variation in identity scores across self-selected activities was substantially greater than between-person variability. Mean level identity scores were not statistically different across activity types, but boys did report significantly lower expressive identity scores in instrumental activities (school, job) than did girls. After controlling for background variables, general activity participation and self-defining activities were significantly related to wellness. Expressive identity mediated the relations between self-defining activities and wellness. Implications for interventions promoting activity involvement and identity formation are discussed.