Recent sporadic distribution in Japan of Trichophyton mentagrophytes skin infections among laboratory rats or guinea pigs is reported. Six cases of laboratory infection by the fungus in humans who came in direct contact with the affected animals are also presented. Mating experiments with the causative fungi isolated from the animal and human skin lesions revealed that all but one sexually reactive strains belonged to the "+" mating type of Arthroderma vanbreuseghemii. A retrospective epidemiologic study ascertained that most of the affected rats were those derived from the same breeding colony of a certain experimental animal producing-co-operation in Saitama prefecture, where it had been produced by conventional methods of specific pathogen-free parental colony maintained in a barrier system. These two pieces of evidence suggest a possible transmission of the etiologic agent from some healthy carrier or infected hair-droppings in the environment to the breeding colony, during its breeding, or maintenance by the producer.