Objective - The purpose of this study was to determine if class size in a health-related fitness course affects physical activity behaviors of urban, commuter university students. Methods - Survey questions related to physical activity behaviors were administered to 653 students enrolled in small (~40 students) or large (~100 students) health-related fitness courses (small experimental, SE or large experimental, LE) designed to promote positive physical activity patterns or a control group (C). The data were analyzed using ANOVA and Chi-Square tests. Results - Following the 15-week intervention, patterns emerged suggesting that the SE was more effective than the LE in eliciting increases in the quantity and quality of physical activity. When the data were dichotomized to reflect compliance with minimal standards, the SE group had a significantly (p<.05) greater proportion of subjects compared to the LE group who reported exercising 150 + minutes/week (14.6% vs 5.3%, respectively) and who achieved 450 + MET . minutes/week of physical activity (17.3% vs 4.6%, respectively). For the subjects classified in the active stages of change (action and maintenance), only the LE group (7.4% change) reported a significant increase (p<.05) in the proportion who were active, though a trend for the SE group (9.6% change) to increase (p=.066) was observed. Regarding physical activity enjoyment, there were no differences found among groups. Conclusion Based on the findings from the present study, class size in a 15-week health-related fitness course appears to affect changes in physical activity behaviors. Therefore, if a primary goal of health-related fitness courses is to increase participation in physical activity, it would be a prudent policy to limit enrollment in such courses.