The standard tube agglutination test (STAT) and the complement fixation test (CFT) were used to assess the seroprevalence of antibodies to Brucella spp. in caribou (Rangifer tarandus) from three populations on Baffin Island, Canada. During late winter from 1983 to 1986, sera from 17 of 40 North Baffin (43%), 11 of 33 Northeast Baffin (33%) and 12 of 82 South Baffin (15%) adult caribou had antibodies in the STAT at 1:50 or the CFT at 1:5. Seroprevalence increased as caribou matured with one (4%) of 25 calves, four (13%) of 31 yearlings, and 40 (26%) of 155 adult caribou being positive. However, seroprevalence did not differ with sex in any age class. Positive antibody titers were higher in adult females sampled in May, 3 to 4 wk before parturition, than in adult females sampled in late March and April. The strength of positive titers did not differ with the time of sampling among adult males. Pathologic signs of brucellosis were found in three (13%) of 23 caribou that were assumed to have active infections (caribou with CFT titers > 1:160). Brucella suis biovar 4 was isolated from 24 (60%) of 40 caribou from which lesions were submitted. Between 1986 and 1990, the annual incidence of reported human (Homo sapiens) cases averaged 3.4 (34:100,000) on Baffin Island.