Industrially used sake yeasts (Saccharomyces cerevisiae) such as Kyokai no. 7 and no. 9 hardly sporulate under any of the nutritional conditions examined to date. However, through acquisition of either sensitivity to ethionine or the ability to grow at 35 degrees C in the presence of beta-alanine, these sake yeasts were found to regain the ability to form asci with 4 viable spores at high frequency. These methods for the mutant-selection may facilitate the genetics of industrially used yeasts and their breeding by hybridization.