When Mycobacterium convolutum R22 was grown on the n-alkanes C13 through C16, the predominant fatty acids were of the same chain length as the growth substrate. Cells grown on C13 through C16 n-alkanes incorporated between 15 and 85 pmol of acetate per pg of lipid into the fatty acids, whereas acetate-or propanegrown cells incorporated 280 and 255 pmol of acetate per ,ug of lipid, respectively.In vivo experiments demonstrated that hexadecane, hexadecanoic acid, and hexadecanoylcoenzyme A (CoA) all inhibited de novo fatty acid synthesis. Hexadecanoyl-CoA was the most potent inhibitor. Hexadecane and hexadecanoic acid inhibited acetyl-CoA carboxylase by up to 37 and 39%, respectively, at 1 mM. Hexadecanoyl-CoA inhibited the enzyme activity by 65% at 50 uM. Cells that were grown on C14 through C1s n-alkanes had about 25 times less acetyl-CoA carboxylase activity than did cells grown on acetate or propane, suggesting repressed levels of the enzyme. Hexadecaneor pentadecane-grown cells were found to have 5 to 10 times more intracellular free fatty acid than cells grown on acetate, propane, or ethane.Bacterial and yeast cells growing on longchain n-alkanes (C14 through C18) as sole sources of carbon and energy contain a large proportion of fatty acids that are the same carbon chain length as the hydrocarbon substrate (6, 7, 9a, 14, 20, 23, 25). King and Perry (11) have demonstrated that in hexadecane-grown Mycobacterium vaccae, the predominant fatty acids tetradecanoic, hexadecanoic, and hexadecenoic acids were derived directly from the carbon skeleton of hexadecane. Similarly, n-octadecane served as the immediate precursor for tetradecanoic, hexadecanoic, hexadecenoic, octadecanoic, and octadecenoic acids. Makula and Finnerty (16) demonstrated that Acinetobacter HO1-N accu-