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Brown bears (Ursus arctos) in southcentral Alaska spent an average of 201 days in winter dens. Males spent the least time in dens (mean = 189 days) and parturient females the most (mean = 217 days). Females with cubs of the year and females pregnant at den entry spent the least amount of time out of dens (158 and 164 days, respectively) and males the most (180 days). No difference in den entrance date based on sex or reproductive status was observed. Mean den entrance date was 14 October. Entrance date differed between years, early entrance appeared associated with berry crop failures and colder weather. Mean date of exit from dens was earliest for males (23 April) and latest for females with newborn cubs (15 May). Exit dates also varied between years with late exits correlated with colder weather and persistent snow cover. Dens used by brown bears in this area were excavated, no unmodified natural cavities were used. These dens collapsed during spring and summer precluding reuse. Some individuals dug dens in the same general area from year to year; mean distance between den sites used in successive years by all bears was 6.1 km. Characteristics of den sites and sizes of dens are described. Typically dens were dug at higher elevations and on the periphery of home ranges used during summer and fall. Upon exit, most bears moved to lower elevations but females with newborn cubs tended to remain in the vicinity of den sites. Available data suggest this behavior reduces loss of newborn cubs to predation by other bears. Compared to a sympatric population of black bears (Ursus americanus), brown bears denned at higher elevations, spent less time in dens, and entered dens earlier. Den exit dates were similar. Dimensions of brown bear dens were not significantly larger than excavated black bear dens and mean date of emergence from dens was about the same. A proposed hydroelectric project in this study area would likely have reduced black bear populations through impacts on black bear denning habitat. The project would have had only indirect impacts on brown bear denning habitats.