Self-determination theory (SDT; Deci & Ryan, 1985) has proven to be a useful framework for the investigation of a wide range of health-related behaviors. Levels of self-determination are delineated on a continuum ranging from amotivation (complete lack of motivation) to intrinsic motivation (engaging in an activity out of interest and enjoyment). Four levels of regulation lie between these two extremes: external, introjected, identified, and integrated. As individuals move from lower levels of regulation (external) to more self-determined levels (identified and integrated), they are more autonomous (i.e., acting out of their own volition). According to SDT, self-determined individuals are more likely to initiate and sustain a target behavior, such as engaging in exercise, than those who are less self-determined, or feel that they are being controlled. From this perspective, it becomes important to understand how practitioners can structure the instructional climate to foster higher levels of self-determination. This, in turn, should lead to high levels of adherence to an exercise program. The purpose of this study was to investigate the relationships between self-determination, perceptions of an autonomy supportive climate, and improvement in cardiovascular fitness. Participants were 91 (31 males and 60 females) undergraduate kinesiology students enrolled in a course that required them to participate in a semester-long personal fitness program. Improvement in cardiovascular fitness was assessed using pre-and post-test distances on a 12 min run. Participants completed surveys during the last week of the semester to assess their perceptions of the climate (Williams et al., 1996) and levels of self-determination (SRQ-E, Ryan & Connell, 1989). Data were analyzed using simple correlations among levels of self-determination and the perceived climate. A blocked multiple regression was used to examine the relationship between self-determination and improvement in cardiovascular fitness. Specifically, the pretest score from the 12 min run was entered in the first block to account for the variance associated with beginning fitness levels. Subscales from the SRQ-E (external, introjected, identified, and intrinsic) and the perceived climate were entered in the second block. Higher levels of identified regulation and intrinsic motivation were positively associated with the perception of an autonomy supportive climate. Introjected regulation entered the regression model as a significant negative predictor of improvement in cardiovascular fitness. These results suggest that an autonomy supportive climate fostered higher levels of self-determination, which in turn led to improved fitness. They also suggest that focusing on exercise as an obligation may actually serve to hinder engagement.Keyword(s): active participation, exercise/fitness/physical activity, research