Several years ago we accidentally observed a bright-red fluorescence of a lesion of the genitocrural area under the Wood light. The lesion was a dry, reddish-brown plaque with sharply demarcated borders, no papules or vesicles, and slight, powdery scaling. Clinically, the lesion was typically that of erythrasma, and microscopic examination of the skin scrapings under oil immersion showed short, delicate, branching filaments and small coccoid forms characteristic of Nocardia minutissima, the causative agent of erythrasma. Repeated examinations over a period of a month, with care taken to avoid any local applications so as to obviate the possibility of fluorescence due to foreign substances such as medicaments, did not show any change in the appearance of the fluorescence. We have since used the Wood light on all lesions of the genitocrural and axillary areas resembling erythrasma or tinea cruris. No lesion of tinea cruris, proved by microscopic examination, or of