Little is known about nest use by flying squirrels (Glaucomys) in partially harvested forests, especially for northerly populations where cavity use is prevalent. We used radiotelemetry to examine nest use by 24 southern flying squirrels (G. volans) in 2003 in logged and unlogged hardwood forests, and by 18 northern flying squirrels (G. sabrinus) in 2004 in conifer forests, in Algonquin Provincial Park, Ontario, Canada. Of 76 nest trees used by G. volans, 71% were in declining trees and 22% were in snags. Sixty tree nests used by G. sabrinus included 28% snags, 46% declining trees, and 25% healthy trees, although nearly one-half of nests of G. sabrinus that were used on more than 3 occasions were in snags. G. volans used larger-diameter trees and American beech (Fagus grandifolia) more often than expected by chance, whereas G. sabrinus used trembling aspen (Populus tremuloides), white birch (Betula papyrifera), and yellow birch (B. alleghaniensis) more than expected by chance. Both species used a high proportion of cavity nests, few external nests, and trees that were decayed or diseased. We found indications that nest supply was limited in recently harvested sites, where there were fewer cavity trees and snags; however, G. volans may compensate by using abandoned yellow-bellied sapsucker nests and by nesting in aggregations. Hardwood snags and decaying trees appear to provide crucial nesting habitats for both squirrel species, particularly for females.