Hyphal fusions have been recognized as an important character of dermatophytes.In Microsporon audouini, M. lanosum, and Trichophyton gypseum hyphal fusions: (1) are formed between hyphae of one and the same mycelium isolated from a single patient, (2) are formed between any two mycelia of the same species isolated from two different patients, and (3) are not formed between a mycelium of one species and a mycelium of another species.The occurrence or non-occurrence of hyphal fusions between hyphae of two mycelia of different origin may be applied as a criterion for identifying species of dermatophytes whose specific nature is uncertain.