Winter records of bats in the Pacific Northwest are extremely limited (Barbour and Davis 1969). Of the 15 bat species other than Plecotus townsendii in Oregon and Washington, at least two are considered migratory. Five species (Eptesicus fuscus, Myotis californicus, M. Iucifugus, M. volans, Antrozous pallidus) are known to hibernate in the eastern portion of their ranges (Barbour and Davis 1969; Twente 1960). Marcot (1984) reported records for hibernating Myotis evotis in northern California, Senger et al. (1974) noted five species of hibernating myotids (M. californicus, M. evotis, M. Iucifugus, M. volans and M. yumanensis) in Washington, Genter (1986) observed hibernating M. Ie.;bii in Idaho, and Jewett (1955) recorded Tadarida brasiliensis from buildings in southwestern Oregon. Most hibernacula contained relatively few bats (range = 1-41) when compared with those of the eastern United States (Barbour and Davis 1969). The reports were from studies concentrated in small geographical areas and involved a small number of caves. Twente (1960), who investigated over 600 potential hibernacula in Utah, speculated few hibernating bats were noted because of warm temperatures in the caves. Results of Genter (1986), Marcot (1984), and Senger et al. (1974) suggest that hibernacula in the Pacific Northwest may contain larger bat populations than reported by Twente (1960). This prompted us to conduct a more extensive search that included most of Oregon and Washington. We defined winter as the period from 1 November to 1 April. During winters of 1982-1989, we searched more than 650 caves and mines and 70 buildings in Oregon and Washington. Seven of the caves were previously surveyed by Senger et al. (1974). Most bats were easily identified from observable external characters (Hall 1981). Closely related species (M. Iucifugus, M. yomRnensis; and M. thysanodes, M. evotis) were collected if positive identification could not be made in the field. In addition, we obtained winter records of museum specimens collected in Oregon and Washington from American Museum of Natural History (AMNH), the National Museum (USNM), the University of Puget Sound (PSMNH), the University of Washington (UW), Southern Oregon State College (SOSC), the San Diego Museum of Natural History (SDMNH), Oregon State University (OSUFW), the University of Michigan (UMMZ), and Walla Walla College (WWC). We located 174 bats of seven species at 26 sites (Table 1). M. volans (124) and M. Ieibii (36) were encountered more than any other species. Numbers of bats were