The oxidation of both saturated and unsaturated fatty acids by a water isolate and a pathogenic leptospire is reported. Evidence is presented for the existence of an adenosine triphosphate dependent fatty acid activating enzyme in the water isolate, and measurement of the reactions of β-oxidation in cell-free extracts of both strains is described. It was concluded that fatty acid degradation by β-oxidation constitutes a major catabolic pathway inLeptospira.