Somali women living in the US are at increased risk for chronic health conditions due to changes in lifestyle following immigration. Numerous barriers to physical activity have been reported in this population. Behavioral theory may inform the design of successful health interventions. We explored in focus groups the behavioral determinants of physical activity (theory of planned behavior, self-efficacy) among Somali women (N = 30). We found that most (two-thirds) subjects were sedentary, although women who had lived in the US for 10 years or longer were more likely to be active. Somali women recognize the health threat of physical inactivity, including high rates of obesity. Moral norms appear to be the major barrier to physical activity, due to prohibitions against exercising in public or in Western-style clothing. Taking moral norms into consideration should allow for the design of culturally-appropriate exercise programs that can address a major health threat in this vulnerable population.