A strain of Microsporum cookei, which had been isolated by the present authors from the soil attached to the hoofs of a horse in Japan, failed to produce cleistothecia when cultured singly on the sterilized soil to which had been added keratin bait.
Mating of this strain on soil cultures with another strain of the same species originated from the soil of the United States, which had been proved to be no cleistothecium-forming strain, induced the production of cleistothecia.
Soil cultures of single ascospore strains and their parent strains singly and in pairs of all possible combinations showed that the species was heterothallic. These results are in agreement with those already reported by Ajello.
The morphological characteristics of the cleistothecium observed by the present authors were identical to those of Nannizzia cajetana Ajello, 1961.

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