Although it is known that southern flying squirrels (Glaucomys volans) nest in larger groups to offset the energetic costs of low temperatures, the influence of other variables on aggregation size remains relatively unknown. Therefore, the influence of environmental variables and individual characteristics of G. volans on nest box aggregation size was studied from 1992–1998 at the Savannah River Site in South Carolina. Over the study period, 5859 occupied boxes were observed, representing 11,238 captures and 2671 unique individuals. The majority of occupied boxes (60.3%) contained solitary individuals, while 26.1% contained aggregations (≥2 individuals) of adult and/or subadult individuals and 13.6% contained litters. Although ambient temperature is widely believed to be the most important variable influencing aggregation size of G. volans, the final model explaining nest aggregation size for G. volans included not only minimum daily ambient temperature, but also Julian date and individual G. volans characteristics of sex, age, and reproductive status. Minimum ambient temperature was inversely related to aggregation size, with larger aggregation sizes associated with winter months, although aggregation size sharply increased between the months of Aug.–Sept. The retention of Julian date in the final model, in addition to minimum ambient temperature, indicates that behavioral shifts due to seasonal changes, independent of temperature, should be considered. Nestlings were associated with larger aggregation sizes, as were inactive and lactating females and inactive males. However, sex alone did not significantly contribute to the final model. This study demonstrates that in addition to temperature, there are a suite of predictors of nest aggregation size for G. volans that should be considered when evaluating nest box occupancy for this species.