1. The properties of a transport system for the uptake of fatty acids into Escherichia coli cells are described. The system is induced under the same conditions as the enzymes involved in fatty acid degradation. I n accordance with the chain-length specificity of the acyl-CoA synthetase, only fatty acids with more than eight carbon atoms are taken up. The reduced rate of uptake of fatty acids in mutants with various lesions in fatty acid degradation indicates that transport and metabolism are tightly coupled processes. The absence of an efflux reaction and the inability of a mutant lacking the acyl-CoA synthetase to take up fatty acids support the hypothesis that this enzyme is involved in a vectorial acylation during the permeation process.2. Mutants lacking one or all of the enzymes enoyl-CoA hydrase, b-hydroxyacyl-CoA dehydrogenase and thiolase, when assayed with substrates having four carbon atoms, can partially degrade long-chain length and medium-chain length fatty acids. This suggests that there are at least two types of enzyme activity in the cell which differ in their specificities for acyl-CoA substrates of various chain-lengths.3. A new old locus situated between the genes proA and proB on the E . coli linkage group is described. Lesions in this gene make the cell unable to degrade fatty acids of any carbon number, and must therefore lack an element common to the degradation of all fatty acids. Fatty a,cids taken up by these mutants are exclusively incorporated into membrane phospholipids.An inducible system for the synthesis of the enzymes of fatty acid degradation has been described in Escherichia coli [l-41. Since the enzymes are formed in response to the presence of fatty acids in the growth medium the cells must be able to take up fatty acids. The properties of such a transport system are described in this communication. Moreover, a further physiological analysis of mutants unable to degrade fatty acids (old mutants) is presented. Previously described old mutants with lesions in the metE-rha region, which lack one or several enzymes of the @-oxidation cycle, are still able to partially degrade oleate. For three out of the four reactions of the cycle which have been studied, the results suggest the presence of enzymes with different chainlength specificities for their respective substrates This paper is dedicated to Prof. Dr. Feodor Lynen on the occasion of his 60th birthday.Enzymes.