A temperature-sensitive osmophilic mutant of Zygosaccharomyces rouxii, OS15, was isolated, which required high salt or sugar concentration for growth above 30 degrees C. Cell viability at 35 degrees C in the presence of NaCI was higher than in the absence of NaCI, and a survival ratio of the mutant cells after incubation at 55 degrees C was also higher in the presence of NaCI than NaCI-free condition. Furthermore, resistance to UV light, hygromycin B and. geneticin was improved in the presence of NaCI. There was no difference between the patent and the mutant in fatty acid saturation and microscopic cell shape under NaCI condition.