Background: Previous interventions to increase physical activity among middle school students have not produced long-term results. Often, students lack the self-regulation skills needed to support long-term adherence to physical activity. Purpose: The purpose of this study was to conduct a formative evaluation of a self-regulation based physical activity intervention (MyFit) targeting middle school students. The evaluation addressed 3 foci. The first focus was student learning and the use of self-regulation strategies for physical activity. The second was teacher perceptions of the feasibility of the MyFit program. The final focus was student perceptions of the acceptability of the MyFit program. Methods: Seventeen lessons were developed to target self-monitoring, social support, environmental aids, reinforcement, self-efficacy, and tailoring. A one-group pretest-posttest design was used. Results: Students had sufficient knowledge test scores and reported significant increases in the use of self-regulation skills. The teacher and students also provided useful feedback for the refinement of the MyFit program. Discussion: This study provided valuable data for MyFit content and delivery refinement. Translation to Health Education Practice: Formative evaluation methods were useful to refine the MyFit curriculum. The MyFit curriculum is useful for school-based practitioners as a way to increase self-regulation skills to improve adherence to physical activity. (Contains 2 tables and 1 figure.)