AbstractHouse dust samples from 7 families of 13 patients with tinea due toT. mentagrophytes(5 patients),M. gypseum(1 patient) andM. cants(6 patients) were cultured several times during their clinical courses. In all cases, the numbers of colonies and positive culture plates of the etiologic dermatophytes decreased in number during tinea treatment.T. mentagrophyteswas not isolated from house dust any more by the time the patients were cured;M. caniswas still isolated after the tinea lesions had healed. These results seem to indicate: 1) cat fur, the source of infection, is difficult to remove by simple cleaning of the house (or clothing). 2) The parasitic form of the fungus is much more resistant and the numbers of dermatophytes in the fur are greater than those in the scales of the patients.