A series of environmental and clinical isolates of Sporothrix schenckii being studied in our laboratories under standard conditions of temperature, humidity and media, displayed, in some of the isolates, large dematiaceous hyphae. The large hyphae could be seen to produce the fine hyphae associated with S. schenckii on which typical microconidia developed. Typical microconidia also developed occasionally on the large hyphae, and strands of the unusually large hyphae also were seen to form arthroconidia. Some strains formed large, thin-walled cells on thin and/or thick hyphae resembling the 'balloon cells' seen in mature colonies of Trichophyton tonsurans. All strains which have demonstrated these various and unusual structures histopathologically produced typical sporotrichosis in laboratory animals. When reisolated from these animals, the cultures again exhibited the unusual structures, as well as those typical of S. schenckii.