Between 1982 and 1999 blood samples were collected from 500 polar bears ( Ursus maritimus ) captured in the Beaufort and Chukchi seas, to determine the seroprevalence of Brucella species, Toxoplasma gondii , and Trichinella species infections. The bears were classified into four age groups, cubs, yearlings, subadults and adults. Brucella and Toxoplasma antibodies were detected by agglutination (a buffered acidified card antigen and rapid automated presumptive test for brucellosis and a commercial latex agglutination test for toxoplasmosis); an ELISA was used to detect Trichinella antibodies. The overall seroprevalence of Brucella species was 5 per cent, and subadults and yearlings were 2·62 times (95 per cent confidence interval 1·02 to 6·82) more likely to be seropositive for Brucella species than adults and their cubs. The antibody prevalence for Toxoplasma gondii was 6 per cent, and for Trichinella species 55·6 per cent. The prevalence of antibodies to Trichinella species increased with age (P<0·001).