1. In March, 1925, the writer obtained a new form (M1) of Sclerotium Rolfsii in the culture of origin 44. Its mycelia are fine and slender measuring 5.Oμ in width and take on brown color with cottony ruster. Its sclerotia are extremely irregular and large with rough surface. Colored outer part of the sclerotia consists of three or four layers measuring 33μ in thickness. It shows one-sided aversion to original and mutual aversion to other strains of the fungus. Loss of pathogenicity, production of spores in cultural media and formation of blister-like sclerotia in early stage of their development are the most characteristics of the fungus (M1). 2. In July, 1925, the writer also obtained a new form (M2) of the fungus in the culture of origin 6. It is the same in all respects as original, with exception of the phenomenon of mutual aversion, which occurs between them. 3. M 1 and M2 bred true during repeated subsequent transfers. They have been grown on different kinds of media and under differerent conditions without losing their constant characters, especially for cultural, morphological and averting characters. Every part of mycelia taken as an inoculum shows the same characters on its subculture. 4. Mutant in fungi is supposed merely as phenotypes and interpreted to be due to segregation or hybridization. So 'saltant' or 'transmutant' is applied to fungi distinguishing from 'mutant'. Considering Sclerotium Rolfsii the mycelia fuse together and a binucleated cell is formed. In the case the fusion takes place only between the same strains of the fungus; if the strains are different they avert mutually leaving non-growth-region of the mycelia. Therefore nuclei in the binucleated cells are originated from the same strains and they should be the same in nature. So new forms M1 and M2, different from originals are asserted to be true mutants caused by genotypic change, not by segregation or hybridization. 5. The mutants M1 and M, are of importance in proving the possibility of being so many strains in the fungus, in asserting the perfect form of the fungus to be Hypochnus and in offering an example of true mutant in fungi comparable to bud variation in higher plants.
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