1. 1. Phospholipids, total fatty acids, sterols and hydrocarbons were determined in respiratory-competent and respiratory-deficient cells of Saccharomyces cerevisiae grown aerobically and anaerobically. 2. 2. In a respiratory-competent strain, the previously-reported reduction in the content of phospholipids, fatty acids, and sterols of cells grown anaerobically on glucose in medium supplemented with ergosterol and Tween 80, relative to aerobically-grown cells, was confirmed. Anaerobically-grown cells from medium lacking ergosterol contained more phospholipids and fatty acids, and from medium lacking both ergosterol and Tween 80 contained more phospholipids, than cells from the supplemented medium. 3. 3. Changes in the lipid fractions during respiratory adaptation of anaerobically-grown respiratory-competent yeast were followed. The level of phospholipids rose during the first 2 h of adaptation, the level of other fractions rising during several hours approximately parallel with the development of respiratory activity. 4. 4. When anaerobically-grown respiratory-competent yeast was aerated in the presence of acriflavin or chloramphenicol, no respiratory adaptation took place. However, the content of phospholipids and of total fatty acids increased in a manner similar to that in cells during respiratory adaptation, whereas the content of sterols increased less. The cells aerated in the presence of cycloheximide had no respiratory ability, the content of phospholipids was similar to that in the cells aerated with acriflavin or chloramphenicol, the content of fatty acids was intermediate between aerobically- and anaerobically-grown cells, and the content of sterols was markedly reduced and approximated that of non-aerated anaerobically-grown cells. 5. 5. The respiratory-deficient mutant, derived from the competent strain, showed the same pattern of lipid composition as the original strain. The amount of lipids was reduced in anaerobically-grown cells. The lipid composition of aerobically-grown mutant cells was the same as that of aerobically-grown wild yeast, except for the phospholipid content which was lower in the mutant.