Objectives: The present study explored the experience of introjected regulation (i.e. a controlling motivational regulation in which people act due to internal pressures that are regulated by contingent self-esteem; [Ryan, R. M., & Deci, E. L. (2000). Intrinsic and extrinsic motivations: classic definitions and new directions. Contemporary Educational Psychology, 25, 54-56]) in relation to sport and exercise in mid-adolescence. Methods: Adolescents reporting strong introjected regulation of sport and/or exercise relative to their peers were identified using quantitative questionnaires, and invited for interview. Semi-structured interviews were recorded with 10 boys and 8 girls (mean age 14 years), transcribed verbatim, and analysed using an interpretive phenomenological approach. Results: Introjected regulation accompanied high levels of self-determined motivation, and was associated with high levels of physical activity in the present sample. Two major themes emerged: (i) gender differences in the basis for introjected regulation; and (ii) differences in the reasons and goals underpinning self-determined versus introjected regulations for exercise. In boys, introjected regulation was largely related to social factors, such as avoiding social disapproval and attaining ego enhancement. Girls rarely exercised with their friends, and introjected regulation more commonly reflected the partial internalization of a health and fitness rationale. In many cases, self-determined and introjected regulations were underpinned by different goals or reasons, supporting the importance of assessing an individual's multiple motives towards activities. Conclusions: Introjected regulation for exercise was associated with higher than expected levels of participation in sport and exercise, regardless of whether it was founded on contingent self-worth, or the partial internalization of adaptive reasons for exercise. The implications of social control on future exercise participation are discussed. © 2008 Elsevier Ltd. All rights reserved.