To study the northern flying squirrel (Glaucomys sabrinus) in the mountains of Virginia, we monitored 349 nest boxes at 26 sites from October 1985 to May 1996. The northern flying squirrel was captured 112 times at 6 of these sites; 78 individuals were ear-tagged, 11 neonates were not ear-tagged, and 6 escaped. Twelve northern flying squirrels were recaptured at least once. Most of the individuals (84.8%) were captured at 3 sites in Grayson and Smyth counties. The remaining individuals were captured at a fourth site in Grayson County and 2 sites in Highland County. The northern flying squirrel is now documented from 7 sites in 3 Virginia counties. The northern flying squirrel occupied 28 of the 349 nest boxes. The mean number of squirrels/occupied box was 2.1 (range= 1-5). The sex ratio of adult squirrels was 1.4 males/female. The juvenile to adult female ratio was 1.2 young/adult female during the breeding season. Eleven litters were observed between 1987 and 1994 and mean litter size was 2.5 (range=1-4). Mean body mass of adult males and adult females was 109.8 g (range=93-126) and 120.6 g (range=98-141), respectively. Reproductive activity was first recorded in December for males and in March for females. Mean tail length for squirrels in Grayson and Smyth counties was 128.3 mm, suggesting this population is more closely related to G. s. coloratus than to G. s. fuscus.