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approved: The causes of natural mortality and disease in free ranging black bears, Ursus americanus, in California, Oregon, and Washington are poorly known. Life history components, such as scavenging and overlapping habitat with many species of carnivores, potentially expose bears to a wide range of infectious disease agents. To date, no disease has been identified that appears to greatly influence black bear population dynamics. The objectives of this study were to determine the prevalence rates of exposure to selected infectious disease agents in black bears at six study sites of California, Oregon, and Washington, and to assess if age, sex, study area, or year of sampling are related to the prevalence of specific diseases. One hundred and ninety nine black bear serum samples were collected between 1993 and 1997 and tested for selected viral and bacterial disease agents. Antibody prevalence was 0% for bluetongue virus, 12.6% (24/190) for Borrelia burdorferi (Lyme disease), 0% for Brucella spp., 0% for Dirofilaria immitis (heartworm), 4.8% (8/165) for canine distemper virus, 4.5% (9/198) for Ehrlichia equi, 0% for epizootic hemorrhagic disease virus, 9% (8/88) for Francisella tularensis (tularemia), 1.8% (3/165) for canine infectious hepatitis virus, 2.5% (5/198) for Trichinella spiralis, 45% (89/198) for Toxoplasma gondii and 5.5% (11/198) for Yersinia pestis (plague). Prevalence differences were observed among study sites. Lyme disease and plague antibodies were detected only in black bears from California and Oregon. E. equi antibody detection was highest from California bears. This is the first report of E. equi in the Ursidae family, and the first report of morbillivirus in black bears. Redacted for Privacy These data do not support the relationship reported in other studies of rising prevalence rates with increased age of bears. The potential implications of diseases transmitted by translocated bears or re-introduced sympatric carnivores should be considered before management decisions are made.