OBJECTIVE:Within exercise class settings, group cohesion has consistently been found to predict adherence behaviors, and has been identified as a salient target for intervention-based initiatives. Drawing upon theorizing from the field of group dynamics, exercise class cohesion is often conceptualized as a dynamic construct that requires several classes to form and once it is formed, continues to change over time. Despite the salience of this "dynamic" contention for informing physical activity interventions, this theorizing has yet to be empirically tested. METHOD:In this study a multilevel modeling framework was used to examine changes in exercise class cohesion over time. Exercisers (N = 395) completed measures of cohesion following the second, fifth, and eighth classes of their respective programs (N = 46). RESULTS:Mean levels of social cohesion changed significantly over time whereas mean levels of task cohesion did not. These patterns were largely consistent across persons and groups. CONCLUSIONS:These findings suggest that within group-based exercise programs social and task cohesion possesses different levels of dynamism, and that this dynamism (or lack thereof) might have important implications for future research and interventions involving physical activity groups.