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The temperature limits of growth of three psychrophilic yeasts, Leucosporidium frigidum, L. geZidum and L. nivalis, were examined. All species grew well at subzero temperatures (- I "C). The maximum temperature of growth was significantly higher with glucose as substrate ( I 8 to 20 "C) than with ethanol (17 to 18 "C). There was a positive correlation between the growth temperature and the degree of fatty-acid unsaturation of the cell lipids; the lower the temperature, the greater the degree of fatty-acid unsaturation. At subzero temperatures (- I "C) with ethanol as substrate, 90 % of the total fatty acid was unsaturated with linolenic (35 to 50 %) and linoleic (25 to 30 %) acids predominating. At temperatures close to the maximum for growth, linolenic acid accounted for less than 20 % of the total fatty acid and oleic (20 to 40 %) and linoleic (30 to 50 %) acids were the major com-ponents. Difference spectra of intact cells showed marked changes in the ratios and amounts of individual cytochromes as a function of growth temperature. In L. frigidum with glucose as substrate, the ratios of cytochromes a+a,: b: c at 8 and 19 "C were I: I-I: 2-9 and I: 2-3: 16.7 respectively. Similar changes in cytochrome ratios were noted for L. gelidum, but changes were much less marked for L. nivalis. The temperature effects were interpreted as supporting the view that membrane structure and composition are fundamental to temperature adaptation in psychro-philic yeasts