The first known case of rangiferine brucellosis caused by Brucella suis biotype 4 in a sled dog has been proven by isolation of the organism from the naturally infected animal. The infection was undoubtedly contracted from eating raw, infected barren-ground caribou, Rangifer tarandus granti, in which the disease is enzootic. Limited experience with infections in sled dogs suggests that the disease may run a comparatively mild course in them. Serological evidence indicating the natural occurrence of the disease in wolves, Canis lupus, also is reported for the first time.