American black bears (Ursus americanus) have recolonized western Texas following extirpation in the mid-1900s. Knowledge of winter ecology of black bears is important for conservation and management because denning, parturition, and early cub development occur during this period. We monitored 13 radiocollared black bears for 22 den-years in Big Bend National Park (BBNP) during 1998–2003. All pregnant females (n=6), 2 females with yearlings, 7 subadults, and 1 adult male denned. Three females with yearlings remained active during winter 1998–1999. We located 4 cave dens, 5 ground dens, and 6 rock-pile dens. Volume and elevation of den sites averaged 2.00 m³ (SD=1.51) and 1,800 m (SD=346), respectively. Mean (±SD) dates of den entrance and emergence for 5 pregnant females were 30 December±17 days and 27 April±21 days, respectively, producing a mean denning period of 118±29 days. Pregnant females exited dens later (P= 0.003) and denned longer (P=0.02) than solitary females and males. Black bears in western Texas used den sites located in higher elevations that were remote and highly defensible. A den-habitat model described less than 56 km² of suitable denning habitat within and bordering BBNP. Resource managers should reduce visitor and management activities in and around potential denning areas during winter.