The neutral lipids from whole cells and cell envelopes of aerobic Saccharomyces cerevisiae and anaerobic Sacch. carlsbergensis and the cell walls isolated from the cell envelopes were analysed. The effect of anaerobiosis was particularly clear on the neutral lipid composition of the plasma membrane. Compared to the anaerobic membrane, the aerobic membrane contained more C16:1, C18:1 and other unsaturated fatty acids, more total sterol, more than ten times as much ergosterol and less than one tenth as much squalene, reflecting differences between the aerobic and anaerobic whole cellmthe main sterol in the aerobic membrane ergosterol, was mainly in the free form, whereas zymosterol, 24(28)-dehydroergosterol, epi- or fecosterol and lanosterol were predominantly esterified. In contrast, the anerobic membrane contained small amounts of biosynthetic sterol precursors of ergosterol (mainly esterified), and was clearly richer in saturated fatty acids having a greater variation in chain length and in 18:2 acid. Both plasma membranes contained a considerable amount of triacyglycerols, while the amount of lower acylglycerols was clearly higher in the anaerobic plasma membrane. The lipid composition of both cell walls were relatively similar, consisting mainly of triacylglycerols and lower acylglycerols.