Incidence of brucellosis in elk (Cervus canadensis) on two winter feedgrounds in Wyoming was examined over a 5-year period by testing serum samples using the standard plate agglutination (SPT) buffered Brucella antigen (BBA), rivanol (Riv) and complement fixation (CFT) tests. Thirty-one percent of 1,165 elk were positive by defined criteria. Considering each test individually, only 29% (106) of 370 positive sera would have been classified as reactors by the SPT, 83% (307) by the BBA test and 86% (314) by the Riv test. The CFT would have identified 85% (267) of 332 positive samples on which it was used. Brucella abortus, type 1, was isolated from 17 of 45 elk necropsied. The SPT identified 59% (10) of these as reactors, the BBA test 94% (16) and the Riv test 88% (15). The CFT identified nine of nine (100%) on which it was used. Prevalence of sero-positive animals increased with age. Brucellosis has been present in one of the two elk herds since at least 1930, and the incidence of infection among mature females in both herds was approximately 50% during this study. No single serologic test should be relied upon to diagnose brucellosis in elk.