Chlorella pyrenoidosa cells were grown with 22.5 μM cerulenin (Ce) (a specific inhibitor of fatty acid biosynthesis) for 72 h and the effects on the lipid and fatty acid compositions of the individual lipid classes were examined. Ce-treatment resulted in higher levels of palmitic and linolenic acids, while the level of linoleic acid was strongly reduced. A marked reduction in the galactolipid (GL) content, and especially that of digalactosyl diglyceride (DGDG), was found in Ce-treated cells. Phospholipids (PLs) in Ce-treated cells became richer in phosphatidylethanolamine (PE), whereas the levels of phosphatidyglycerol (PG) and phosphatidylcholine (PC) hardly changed. The monogalactosyldiglyceride (MGDG), DGDG and PC were richer in unsaturated fatty acids in treated cells. Fluorescence polarization of 1,6-diphenylhexa-1,3,5-triene (DPH) indicated more fluid membranes in Ce-treated cells, as assayed on isolated PL multibilayers. The quantitative and qualitative differences in the lipid composition of Ce-treated Chlorella are discussed in relation to herbicide resistance, and confirm the importance of the native composition of thylakoids in the herbicide-binding function.