Escherichia coli grows on long-chain fatty acids after a distinct lag phase. Cells, preadapted to palmitate, grow immediately on fatty acids, indicating that fatty acid oxidation in this bacterium is an inducible system. This hypothesis is supported by the fact that cells grown on palmitate oxidize fatty acids at rates 7 times faster than cells grown on amino acids and 60 times faster than cells grown on a combined medium of glucose and amino acids. The inhibitory effect of glucose may be explained in terms of catabolite repression. The activities of the five key enzymes of beta-oxidation [palmityl-coenzyme A (CoA) synthetase, acyl-CoA dehydrogenase, enoyl-CoA hydrase, beta-hydroxyacyl-CoA dehydrogenase, and thiolase] all vary coordinately over a wide range of activity, indicating that they are all under unit control. The ability of a fatty acid to induce the enzymes of beta-oxidation and support-growth is a function of its chain length. Fatty acids of carbon chain lengths of C(14) and longer induce the enzymes of fatty acid oxidation and readily support growth, whereas decanoate and laurate do not induce the enzymes of fatty acid oxidation and only support limited growth of palmitate-induced cells. Two mutants, D-1 and D-3, which grow on decanoate and laurate were isolated and were found to contain constitutive levels of the beta-oxidation enzymes. Short-chain fatty acids (<C(8)) do not support growth of either the parent strain or the mutants D-1 and D-3. Evidence is also presented to show that decanoate is actively transported by the parent strain and by the mutants.