Although increasing physical activity has been identified as a critical component to improving overall physiologic and psychological health (DHHS, 1996, 2001), only a minority of the U. S. population currently engage in recommended levels of physical activity. Regardless of race or ethnicity, low-income women have the lowest rates of physical activity of all population groups (DHHS,1996; CDC, 2000). Given the demonstrated benefits of being physically active during pregnancy for higher-income women, and the fact that pregnant women are especially concerned about their health, pregnancy affords a window of opportunity to implement interventions to increase low-income women’s physical activity. The purpose of this experimental preliminary study is to determine the effect of an intervention to increase the physical activity of low-income pregnant women to levels recommended by the American College of Obstetrics and Gynecology (ACOG Committee Opinion, 2002), that is, 30-minutes a day at moderate-intensity for most days of the week. A secondary aim is to determine changes in physical activity self-efficacy during pregnancy. Women (n = 50) attending a Planned Parenthood Prenatal Clinic serving uninsured and Medicaid clients will be recruited and randomly assigned to an Experimental or Control Group. Subjects in the experimental group will receive an intervention to increase physical activity during their 13th week of pregnancy. The intervention is based on social cognitive theory (Bandura, 1986) and includes educational material, individual counseling and encouragement, situational and environmental components, and a protocol for walking for exercise. Data will be collected using a Weekly Activity Diary, the SW-200 Digi-Walker Step Counter (Yamax Corp. 2002), the Category Scale for Ratings of Perceived Exertion (RPE; Borg, 1998), and the Exercise Self-Efficacy Scale from the 14th to the 36th week of the women’s pregnancy.  Changes in the frequency, duration, and intensity of weekly walking for exercise, in the number of steps taken weekly, and in reported physical activity self-efficacy between the experimental and control group from the 14th to the 36th week of pregnancy will be determined using multi-level modeling. Data from this study will inform future studies examining the dose-response effects of physical activity on low-income women’s psychological (depression) and physiological (gestational diabetes, maternal hypertension, and fatigue) health outcomes during pregnancy and the postpartum period.