The epidemiological study of a focus of Brucella infection revealed that an outbreak of brucellosis occurred in a small town, and the source of this infection was a domestic cat. As the result of contacts with this cat, six persons, among them three children aged 3, 8 and 12 years, had brucellosis. In all these patients acute brucellosis was diagnosed. Simultaneously with the clinical manifestations of the disease, a rise in antibody titer from 1:50 to 1:1,600 was observed. Brucella cultures isolated from the blood of one of the patients and from the internal organs of the cat exhibited the properties, similar to those of "rodent" strains, i. e. their differential signs permit their classification with B. suis, serovar 5.