Physical activity participation rates in the United States have been in steady decline for the last 25 years, so much so that 60% of youth ages 9-13 years get no physical activity outside of school. This state of inactivity indicates that promoting participation in physical activity at a young age is of importance. For the present study, a computerized intervention based on the Transtheoretical Model and Social Cognitive Theory was designed and implemented in a school setting to study its potential effectiveness on self-efficacy and stage of change for physical activity. Participants (n = 98) were divided into three conditions. Students in the two experimental conditions interacted with a computerized agent program for 40 minutes, once per week for 5 weeks. Students in the control group received the same educational information in written form. Results support the use of computerized human agents for increasing exercise efficacy and health and fitness knowledge. (Contains 1 table and 3 figures.)