Trichophyton gypseum and Epidermophyton floccosum are dual fungi, i.e., composed of 2 distinct constituents associated together in culture, one producing conidia in abundance but relatively scant mycelium, designated as the C or conidial constituent, the other producing fewer conidia and more abundant mycelium, designated as the [image] or mycelial constituent. The [image] constituent arises as a mutation in old cultures of the C type even though the culture is started from a single cohidium. It appears to be genetically stable. Chance segregation of M and C nuclei at the time spores are formed on heteroearyotic mycelium of E. floccosum provides a mechanism for explaining the variation exhibited upon single spore analysis of old cultures. Members of a sp. may represent culturally distinct races which probably correspond to ecological or geographical variants. The naming of spp. on the basis of gross cultural characteristics on standard media is untenable.