1. Observations under filtered ultra-violet light of the available material of Microsporon audouini, M. felineum, Trichophyton gypseum, T. violaceum, T. album and Achorion schoenleini indicate that the species belonging to the genera Microsporon and Achorion produce an intense green fluorescence in the substance of hairs infected by them. Trichophyton-infected hairs appear a paler bluish-white. Normal pigmented hairs appear dark.2. It is shown that the spores and hyphae of the fungus whether within or without the hair are relatively non-fluorescent as contrasted with the substance of the infected hair.3. A fluorescent substance present in hairs infected by M. audouini, M. felineum and Achorion schoenleini is readily soluble in hot water. No such substance could be extracted from the hairs infected by Trichophyton sp., or from normal hairs. It is therefore believed that the presence or absence of the green fluorescence is pathognomonic.