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I located 51 different black bear (Ursus americanus) den sites (28 of males, 23 of females) in Michigan's Northern Lower Peninsula between September 1977 and March 1980. During the 3-winter period, 10 radio-tagged males denned in swamp habitat 24 times, lowland habitat 2 times, and upland habitat 2 times; 7 females denned in swamp habitat 11 times, lowland habitat 5 times, and upland habitat 7 times. Habitat selection for den sites between sexes differed significantly (X2 = 8.29, df = 2, P < 0.002). Of 31 dens visited on foot, 5 were unprotected snow 'nests' lined only with vegetation, 17 were located in depressions or chambers under downed trees or root masses, 3 were in underground cavities, 1 was a nest next to a tree, and 4 were excavated under stumps. The den floor of 1 adult female with 2 cubs contained 2-3 cm of water. Dens of all bears except 1 male were lined with varying amounts of vegetation. The 3 most heavily used, and probably critical winter habitat types were swamp conifers (24 dens for males, 11 for females), upland hardwoods (2 dens for males, 7 for females), and lowland brush and hardwoods (2 dens for males, 5 for females). The average distance of dens from a center of human activity was greater for males (x̄ = 1.26 km) than for females (x̄ = 0.55 km, P < 0.01). Two males denned close to active oil wells (x̄ = 0.32 km), whereas 2 females denned near active snowmobile trails (x̄ = 0.08 km). The activity most disruptive to denning appeared to be human encroachment. Residential and commercial development increased during the study, including land-filling of swamps, construction of wastewater treatment facilities, and expansion of a gravel pit.