THE production of tetraploids in a brewery yeast has been reported by Subramaniam and Krishna Murthy1 and Mitra and Subramaniam2, following treatment with acenaphthene and chrysene. These authors did not present genetic evidence to support their claim, which was based mainly on colony and other gross observations. Cytological evidence has recently been presented for a tetraploid with four chromosomes, derived from a “diploid two chromosome… strain”3. Tobias4 studied a yeast isolated after acenaphthene treatment in terms of survival after X-irradiation, and concluded that the isolate was tetraploid. DeLong and Lindegren5 have characterized certain strains of Saccharomyces cerevisiae as triploid and tetraploid, based on the shapes of survival curves after ultra-violet irradiation. A paper by Lindegren and Lindegren6 has recently appeared, describing the production of tetraploid yeast by crosses of diploids homozygous for complementary mating types. We wish to give a preliminary report on a procedure for the isolation of pure triploid clones of S. cerevisiae.