We constructed protoplast fusants between a temperature-sensitive mutant gal-31 strain of Saccharomyces cerevisiae Kyokai no. 7 of which restrictive temperature was 37°C, and a respiratory- deficient mutant os S. cerevisiae Kyokai no. 9. The fusants were detected and selected on medium plates containing yeast nitrogen base added with 3% glycerol and that added with 2% galactose at 37°C. Four out of 23 fusants, which were obtained at over 10^-5 frequency, showed higher ability of sporulation than their parental strains. We obtained diploid strains among the spore clones isolated from the fusants by the method of random spore plating. In test sake making on a laboratoryscale (100 g total rice), 4 of the diploids produced sake containing higher concentrations of malic acid or isoamyl acetate and ethyl caproate than sake yeast strains Kyokai no. 7 and Kyokai no. 9 while the contents of the general components were about the same for the test strains and the sake ones.