This paper describes a study of the occurrence of T. mentagrophytes var. gran (ringworm) infection in small mammals living in nature and the possibility of the transmission of the aetiological agent from the natural focus of infection to man. The epizootological importance of small mammals is considered from the point of view of their ecological valency, an essential factor in the process of inter-species spread of infection. The concentration of small mammals during winter months in the proximity of human dwellings is due to their search for food and shelter. The barns--the main work place of agricultural workers in winter--were found to contain the highest concentration of small mammals, of the most diverse species and biotopes. They are considered to be the main link in the process of transmission of T. mentagrophytes infection from natural foci to man.