The Siberian flying squirrel (Pteromys volans) is a herbivorous, nocturnal, and arboreal rodent living in boreal coniferous forests. Home-range sizes, movements, and nest-use behavior of Pteromys were studied by radiotelemetry in southern Finland in 1996–1998. Thirty-seven animals were tracked. Average home-range size measured by 100% minimum convex polygons was 59.9 ha for males and 8.3 ha for females. Both sexes concentrated their activities in core areas that represented 9% and 11% of the home-range areas in males and females, respectively. Home ranges of males and females were several times larger than predicted according to body mass. Similarly, home ranges of Pteromys were much larger than in other gliding herbivores. Males especially showed great mobility; the average distance moved from the nest at night was 292 m, and the longest distances recorded were >2 km. A plausible explanation for the large home ranges and great mobility of Pteromys is its gliding ability; both sexes can reach distant parts of the home range for foraging, and males also can reach distant parts for receptive females. Pteromys had several nests, both cavities and dreys (nests in branches of trees), which they changed frequently.