A REVIEW of the medical literature revealed a paucity of information concerning the occurrence of onychomycosis due to the Microsporum genera of fungi. In 1945 Montgomery and Casper1 listed Microsporum lanosum as the cause of onychomycosis in only 1 of 657 cases involving the finger nails at the New York Skin and Cancer Unit during the period 1935 to 1943. An additional 259 cases of onychomycosis of the toe nails were reported in the same article, but no Microsporum species were involved. No details of the one case reported were given in the text. Lewis and Hopper,2 in their textbook, make no mention of Microsporum affecting the nails. The same is true of the Sutton and Sutton3 text. Ormsby and Montgomery,4 in their text, state, "Tinea unguium [is] normally caused by Tricophyton and Epidermophyton floccosum (rarely Microsporum) and less commonly by Candida (Monilia)