The American black bear (Ursus americanus) population on the Bruce Peninsula in southern Ontario, Canada, is small (∼300 bears), genetically isolated from the closest bears in other parts of Ontario because of geography and urban development, and it is at risk because of habitat loss. The Bruce Peninsula is underlain by dolostone, and soils over much of the Peninsula are shallow. The bedrock is karstic with extensive networks of rock fissures and underground drainage systems created by solutional processes. During the nondenning season bears select dense mixed forest and dense deciduous forest stands. From May 1998 to February 2004, we documented the habitat requirements of denning black bears on the Bruce Peninsula and described the characteristics of winter dens in this unique substrate. Thirteen of 30 (43%) dens were located in dense mixed forests, 12 of 30 (40%) were located in dense coniferous forests, and 4 of 30 (13%) were located in dense deciduous forests. Eighty-one percent of dens (25 of 31) were within rock crevices often >2 m deep and ending in a subsurface chamber. Of the remaining dens, 3 were excavated, 2 were under brush piles, and 1 was under a large boulder on a steep talus slope. Twenty-one of 29 dens were located with potential sanctuary trees (>30 cm diameter at breast height) within 30 m of the den. Population viability analysis determined the most effective management action that would ensure persistence of black bears on the Peninsula to be conserving habitat outside of Bruce Peninsula National Park. The importance of dense mixed and dense coniferous stands for denning shown in this study emphasizes that point.