The primary objective of this project was to determine if a walking program based on social cognitive theory and delivered via the Internet could improve fitness levels in women. Fifty-three women were randomized to either an Educational Only condition or a Social Cognitive theory (SCT) condition. The SCT condition emphasized personalized mastery steps and goals to increase walking pace while the Education Only condition primarily provided general physical activity and walking information. The One-Mile Walk Test (Kline et al., 1987), an objective measure of fitness, was conducted to determine if women in the SCT condition had greater improvements in their fitness levels. The psychosocial measures of knowledge, depression, self-efficacy, outcome expectations, social support, and decisional balance pros and cons were administered to determine if changes were evidenced in these psychosocial measures pre to post treatment. Results indicated that both conditions had significant improvements in their fitness levels as measured by their walk test times and estimated VO2max. The SCT condition improved their time on the walk test by 69 seconds at post-test with an increase in estimated VO2max of 2.65 ml/kg/min whereas the Education Only condition improved their time on the walk test by 37 seconds with an increase in estimated VO2max of 1.1 ml/kg/min. Both groups also evidenced change on the self-report measures of stage of change with the SCT condition advancing 1.25 stages and the Education Only condition advancing .76 stages. Total minutes walked per week in the SCT condition increased by 31 minutes to 94 minutes total per week and by 47 minutes to 82 minutes total per week in the Education Only condition. Few of the psychosocial measures evidenced change pre to post. Of note, however, was the significant decrease in self-efficacy seen in the Education Only condition. In summary, both programs were able to make improvements in participants' fitness levels and time spent walking per week. However, the SCT condition, with its guided mastery experiences, had greater fitness improvements that are likely to have a larger clinical health impact.