In this prospective study we applied Self-Determination Theory to investigate the effects of students' autonomous motivation and their perceived learning climate on motivational adjustment, learning achievement, and cardiorespiratory fitness (CF) in a four-month physical education personal conditioning program. 253 urban adolescents (ages 12-14 yrs, 47.9% females) completed measures of Relative Autonomy Index (RAI), Teacher Autonomy Support (TAS), perceived competence, and goal orientation. Fitness knowledge was measured using a validated knowledge test and cardiorespiratory fitness was measured using the PACER test. Learning achievement and CF enhancement were then computed using the arithmetic difference between pretest and post test scores on the knowledge and PACER tests, respectively. A series of multiple regression analyses were conducted. The result revealed that RAI at pretest significantly predicted perceived competence [â (1, 251) = .28, p<.01] and mastery goal orientation ([â (1, 251) = .15, p<.05] at post test. With controlling corresponding scores at pretest, TAS positively predicted the change in RAI [â (2, 250) = .32, p<.01] and mastery goal orientation [â (2, 250) = .28, p<.01]. For CF enhancement and learning achievement, we added an interaction term (e.g., RAI * TAS) after entering RAI and TAS to explore whether students who are widely discrepant in their initial autonomous motivation would respond differently to the learning climate. The result showed that neither RAI nor TAS predicted significant variances in CF enhancement, indicating the impact of self-regulation in physical education on CF was not direct. For learning achievement, TAS predicted significant variance [â (3, 249) = .19, p <.05] but the predictive effect of RAI was not found. After controlling the main effects, the interactive effect between TAS and RAI [â (3, 249) = -.14, p <.05] was significant. In order to clarify the interaction, we separated students on the basis of whether their RAI z-scores were greater than or less than zero. Two subsequent regression analyses with TAS as a predictor were then conducted. The results showed that autonomy support was strongly related to learning achievement within the group of students who were not autonomously motivated for learning in physical education [â (1, 120) = .37, p <.01], but not within the group of students who had high autonomous motivation. Overall, our findings support the importance of self-determined motivation in physical education. Although moderated by students' initial autonomous motivation, PE teachers' autonomy support can enhance students' self-regulated learning and contribute significantly to their learning achievement. Keyword(s): curriculum, middle school issues, physical education PK-12