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From 1983 to 1985, 463 serum samples from 11 species of mammals in Redwood National Park (RNP) (California, USA) were evaluated for antibodies to Yersinia pestis by the passive hemagglutination method. Yersinia pestis antibodies occurred in serum samples from 25 (36%) of 69 black bears (Ursus americanus), one (50%) of two raccoons (Procyon lotor), five (3%) of 170 dusky-footed woodrats (Neotoma fuscipes), and one (less than 1%) of 118 deer mice (Peromyscus maniculatus). Two hundred seventy-three flea pools, consisting of 14 species of fleas, were collected from small mammals and woodrat nest cups. Viable Y. pestis were not isolated from any of the flea pools. Significant between-year variations in the frequencies of seropositive bear or small mammal sera were not observed. A significantly higher frequency of plague antibodies was observed in bear sera taken during September collections. Frequencies of seropositive bear sera did not vary significantly by sex or age group of bears. Significant differences were not observed in the frequencies of seropositive small mammals by forest habitat type in which they were captured. This is the first report of Y. pestis infection in Redwood National Park, and the first detailed report of Y. pestis activity in a temperate rainforest.