The authors examined relations among motivational styles and school adjustment in a sample of 786 7th and 8th grade U.S. students. Specifically, the authors tested the hypothesis that agency beliefs mediate relations between styles of motivational self-regulation (i.e., intrinsic, identified, introjected, and extrinsic) and school adjustment (school grades, school well-being, and positive and negative affect). A structural equation model testing this hypothesis indicated that agency beliefs about one's effort mediate the relations between the styles and positive school adjustment. By contrast, the extrinsic style was not mediated by agency beliefs but reflected adverse low-magnitude direct effects on all of the outcomes except positive affect. Overall, the model strongly predicted school adjustment, and adherence to the identified motivational style was particularly important.