Purpose: To assess the associations of the natural environment with obesity and physical activity in nonmetropolitan areas of the United States among representative samples by using 2 indices of outdoor activity potential (OAP) at the county level. Methods: We used the data from 457,820 and 473,296 noninstitutionalized adults aged over 18 years for obesity and physical activity, respectively, from the 2000-2006 Behavioral Risk Factor Surveillance System. The OAP indices were (1) a recreational opportunity index based on 24 variables related to outdoor physical activity, such as the number of facilities available for walking, biking, hiking, and swimming derived from the 1997 National Outdoor Recreation Supply Information System; and (2) a natural amenities index which was based on physical and social environmental characteristics, such as climate, topographic relief, land cover, and tourism. We fitted logistic regression models using generalized estimating equations to control for county level intracorrelation and tested each index separately to assess its relationship with obesity and physical activity. Findings: Recreational opportunities were higher in areas with greater natural amenities. After controlling for individual-level socioeconomic and demographic characteristics, the prevalence of obesity decreased and propensity for physical activity increased with increasing levels of both recreational opportunities and natural amenities. Conclusions: Multiple indices of OAP based on characteristics of the built, natural and social environments were associated with decreased obesity and increased physical activity in nonmetropolitan areas. Public health interventions should consider the opportunities and limitations offered by the natural environment for promoting physical activity and reducing obesity in rural areas. (Contains 3 tables and 1 figure.)