Southern flying squirrels (Glaucomys volans) are small sciurid rodents that reside in deciduous forests of eastern North America. G. volans consumes primarily seeds and fruits, and is active year-round. During winter when food is sparse, flying squirrels conserve energy by forming aggregations in nest-lined cavities of trees. We compared seasonal changes in resting metabolic rate (RMR), nonshivering thermogenesis (NST), body temperature, and body mass of G. volans nesting communally residing in an outdoor enclosure to squirrels nesting singly housed in an outdoor laboratory. Flying squirrels were tested monthly from November 1997 to November 1998. Average RMR was highest in winter (1.14 ml O₂ g⁻¹ h⁻¹) and lowest in summer (0.73 ml O₂ g⁻¹ h⁻¹); minimal differences occurred among group and solitary nesters. Average RMR ranged from 33% (single) to 38% (communal) lower than predicted by body mass. Average NST was lowest in summer (1.62 ml O₂ g⁻¹ h⁻¹) and peaked in winter (4.70 ml O₂ g⁻¹ h⁻¹), demonstrating an inverse correlation with minimum ambient temperature. Squirrels nesting singly experienced a longer period of elevated NST in winter and higher mean NST year-round than did squirrels nesting communally. Average body mass ranged from a low of 66.9 g in September for both groups to a high of 76.4 g in communal nesters in December and 79.3 g for single nesters in March; average body temperature ranged from 36.3 to 38.9°C (communal nesters, December and May, respectively).