Exercise has important health benefits but a large proportion of the population is physically inactive. We examined the stages of readiness to exercise and their relationship to self-efficacy, the costs and benefits of exercising, and self-report of physical activity in a sample of Rhode Island worksites. Using a three-step model-building approach, exploratory principal components analyses were followed by an examination of the stages of change model with confirmatory structural equation modeling procedures. The model was then examined with longitudinal data. Confirmatory and longitudinal analyses showed an excellent tit between the model and the data. Results indicated that the costs and benefits of exercise and self-efficacy for exercise were related to physical activity only indirectly, through the mediation of stage of readiness to exercise. Structural modeling fit indices revealed that much of the variation and covariation in physical activity was explained by the model. There is the potential to enhance the impact of exercise interventions, by targeting them so as to address factors related to these different stages of readiness.