IT IS only when unusual clinical or mycologic manifestations are present that the diagnosis of pityriasis versicolor (tinea versicolor) is troublesome. The areas most likely to cause difficulty are the neck, scalp, groin and extremities. Mycologic variations occur in patients recently under treatment. In clinical diagnosis, the use of Wood's light is a valuable adjunct, particularly in determining the extent of involvement. In the laboratory, the etiologic agent is easily determined by direct examination of scales mounted in potassium hydroxide or lactophenol (a mixture of phenol, U.S.P., 20 Gm.; lactic acid, 20 cc.; glycerin U.S.P., 40 cc., and distilled water, 20 cc.). Mounts in lactophenol may be examined immediately, without waiting for clearing. The slide may be sealed with some suitable substance and kept for some time. CLINICAL OBSERVATIONS The sites of lesions observed in the 47 cases forming the basis for this study are noted in the accompanying table.