We studied the Prince of Wales flying squirrel (Glaucomys sabrinus griseifrons) in temperate rain forest of southeastern Alaska to provide the 1st quantitative estimates of demography from southeastern Alaska and test predictions of the hypothesis that Sitka spruce (Picea sitchensis)–western hemlock (Tsuga heterophylla) forest is primary habitat for G. sabrinus in southeastern Alaska. We expected that abundance, body condition, productivity, and summer and overwinter survival of G. sabrinus would be higher in spruce–hemlock forest (which typically are the old-growth forests of upland sites [upland-OG]) than in peatland-scrub–mixed-conifer (peatland-MC) forest. Mean values of minimum number of animals known alive and density during autumn were higher in upland-OG than in peatland-MC, and both were about 2 times higher than corresponding spring values. Age and sex composition of the population was similar among years, between seasons, and between habitats. Males comprised a larger portion of the population in upland-OG than in peatland-MC forest. Mean body mass was similar between habitats. Minimum summer survival varied among years and between habitats. Overwinter survival was less varied and similar among years and between habitats. Reproductive females were more abundant in upland-OG than in peatland-MC, but percentage of reproductive females during spring and percentage of juveniles during autumn were similar between habitats. These results support the conclusion that upland-OG forests of southeastern Alaska are primary habitat for northern flying squirrels. Still, squirrel densities in peatland-MC were higher than those reported for several managed and unmanaged forest types in the Pacific Northwest, and some demographic parameters were similar between upland-OG and peatland-MC. In southeastern Alaska, peatland-MC habitat likely contributes to breeding populations of G. sabrinus and reduces risk of viability in managed landscapes.