Ergosterol is a major sterol component of fungal plasma membranes. The effects of disrupting the Saccharomyces cerevisiae SYR1/ERG3 gene, which encodes sterol C-5 desaturase, an enzyme of ergosterol biosynthesis pathway, were markedly different for different S. cerevisiae strains and growth temperatures. The null mutation of SYR1 (⊿syrl) in strain RAY-3 A had only a slight effect on the growth rate at 28°C. However, at this temperature, the same mutation caused poor growth in strain KA-311A and no growth in strain W303-1A. The ⊿syrl disruptant of these strains were able to grow at 37°C, as well as their parental strains. Moreover, the growth of the ⊿syrl disruptant of W303-1A and KA-311A strains were severely inhibited at 16°C. These results indicated that ergosterol is essential for growth at low temperatures, and the effects of the gene disruption are variable by the genetic background. The growth defect at low temperatures appeared to be due to the defect of tryptophan uptake in the ⊿syrl mutants. The ⊿syrl mutants were sensitive to a wide variety of drugs, chemicals, and ions, suggesting that yeast ergosterol is imnortant as oermeabilitv barrier against various chemical stresses. © 1995, Japan Society for Bioscience, Biotechnology, and Agrochemistry. All rights reserved.