Little is known about the denning ecology of American black bears (Ursus americanus) in the Cascade Mountains of western Oregon. Extensive logging during the late 20^th century altered the landscape significantly and may have affected the availability and quality of denning habitat. We visited 104 dens of 54 radiocollared bears during 1993–98 to document den-site characteristics and bear behavior in the Cascade Mountains of western Oregon. We also monitored bears in the spring and fall to estimate denning chronology. In addition, we randomly selected 5-ha quadrats to search for fungal activity and potential den sites. Eighty percent of dens we located were in trees that had cavities created by fungal activity. The remaining dens were located in rocky outcroppings and caves, under logs, or on the ground. We found no selection of dens based on micro- or macro aspect, elevation, or slope. Bears denned more than expected in mature timber with trees that averaged >50.8 cm diameter at breast height (dbh). Mean den entry date for bears in our study was November 20, and mean den emergence date was April 15. Pregnant females entered dens earlier and emerged later than barren females, females with yearlings, and all male age classes. Bears were more likely to abandon dens at lower elevations with little snow accumulation and less secure den structures. Fungal activity was randomly distributed throughout the study area. Fifty-one potential tree and log den structures were found in 27 of 64 quadrats we sampled. This information can help federal and state foresters schedule and design management activities within stands of timber containing denning habitat, aid wildlife managers in setting bear hunting seasons, and help timber cruisers and biologists detect and avoid disturbance of active dens.