due to their complex genetic background, industrial strains are typically unsuitable for genetic engineering (12). Instead, traditional breeding techniques other than genetic engineering, such as naral and chemical/physical mutation, protoplast fusion, genome shuming, and mating, are commonly used for improvement of industrial strains. In addition, techniques other than genetic engineering are usually much more effective at improving traits for which the genetic mechanisms are still unclear, such as acidtolerance and thermo-tolerance. In this study, acid-and thermo-tolerance of the diploid strain WW, which is descended from the flocculating industrial strain KF7 (18, 19), were improved by evolution through continuous fermentation followed by haploidization and mating. Continuous fermentation under acidic or high-temperature conditions was carried out to obtain mutants with improved acid-or thermotolerance. Haploids of these diploid mutants were then obtained through tetrad split to permit mating with each other for further improvement in both acid-and thermo-tolerance and for genetic analyses to identify responsible genes and mutations involved in the phenotype of add-tolerance and thermo-tolerance in industrial suaîns.