Mammalian social organization can vary over ecological time. We experimentally manipulated food resources in enclosed populations of prairie voles (Microtus ochrogaster) to test the hypothesis that food quality influences the potential for group formation. During each field season, populations were started by releasing 5 pairs of prairie voles into each of 8 0.1-ha enclosures. Populations were monitored for 18–19 weeks during each field season. One-half of the enclosures received supplemental food, and the other one-half were unsupplemented controls. Density of voles increased throughout each field season. There were significant increases through time in philopatry and number of groups in both the food-supplemented and unsupplemented treatments, but there were no differences between treatments. Groups formed early in the season, apparently before the need for thermoregulatory benefits. Results are consistent with the hypothesis that the social organization of prairie voles is not flexible in response to changes in food quality but that formation of groups might be a density-dependent response.