The occurrence of tetraploidy in bakers' yeasts was demonstrated by the tetrad analysis of mating type alleles and by the determination of cell-size and cell-weight. It was suggested that the growing behavior of cells, e.g. the cluster-formation, may be related to heterozygosity or homozygosity of mating type alleles rather than to the ploidy. The rate of fermentation of given yeasts in dough and in flour free medium is considered to be under control of a heterozygous genetic constitution, in view of the variation in characters among segregants and among hybrids. In crossing experiments, the characters of hybrids and their segregants proved to be statistically parallel to those of component maters prior to crossing, although accompanied by the unexplainable phenomenon that the rates of fermentation of component maters in dough containing a high concentration of sucrose were reduced when combined in hybrids. “Selection and crossing” is an instrument useful for improvement of the practical characters of yeasts. By this method, the rate of osmo-resistant fermentation was considerably increased over the value of the parent. RD yeasts were extremely low in osmo-resistant fermentation.