The degree of unsaturation of fatty acids was higher in Chlorella vulgaris 11h cells grown with air (low-CO(2) cells) than in the cells grown with air enriched with 2% CO(2) (high-CO(2) cells). The change in the ratio of linoleic acid to alpha-linolenic acid was particularly significant. This change of the ratio was observed in four major lipids (monogalactosyldiacylglycerol, digalactosyldiacylglycerol, phosphatidylcholine, and phosphatidylethanolamine). The relative contents of lipid classes were essentially the same both in high-CO(2) and low-CO(2) cells. After high-CO(2) cells were transferred to low CO(2) condition, total amount of fatty acids remained constant but the relative content of alpha-linolenic acid increased during a 6-hour lag phase in growth with concomitant decreases in linoleic and oleic acids. When low-CO(2) cells were transferred to high CO(2) condition, total amount of fatty acids and relative content of oleic acid increased significantly. The amount of alpha-linolenic acid remained almost constant, while the amounts of palmitic, oleic, and linoleic acids increased. Similar, but smaller, changes in fatty acid compositions were observed in two species of green algae Chlamydomonas reinhardtii and Dunaliella tertiolecta. However, no difference was found in Euglena gracilis, Porphyridium cruentum, Anabaena variabilis, and Anacystis nidulans.