BACKGROUND: Systematic study of human behavior in public parks and specific activity settings can inform policy to promote physical activity in diverse communities. METHODS: Direct observation was used to assess physical activity in public parks in Tampa FL (n=10) and Chicago IL (n=18). Parks were selected from census tracts with high concentrations of white, African-American, and Hispanic populations. Representation from low- and high-income census tracts was also achieved. Physical activity was measured by a modified version of the System for Observing Play and Leisure Activity in Youth (SOPLAY). Activity codes from SOPLAY were transformed to energy expenditure per person (kcal/kg/min). RESULTS: Seventy percent of Tampa and 51% of Chicago park users were observed engaged in sedentary behavior. In both cities, children were more likely than adults to be observed in walking or vigorous activity. In Tampa, parks located in neighborhoods with the highest concentration of Hispanic residents were associated with greatest levels of energy expenditure. In Chicago, parks in neighborhoods with the highest concentration of African Americans showed the highest energy expenditure per person. Gender was associated with physical activity only in Tampa parks. Energy expenditure also varied by activity areas. CONCLUSIONS: More than one half of park users in both cities engaged in sedentary behavior. While differences in park-based physical activity by neighborhood income and racial/ethnic composition were observed, these differences can more likely be attributed to the types of designated activity areas that support physical activity. The study findings suggest that specific configurations of park environments can enhance physical activity in parks.