The mode of degradation of long chain keto acids by two microorganisms was investigated.Escherichia coli K-12converted 12-ketooctadecanoic acid to 4-ketodecanoic acid, accumulating some amounts of intermediates, 10-ketohexadecanoic, 8-ketotetradecanoic and 6-ketododecanoic acids. In contrast,Candida tropicaliscompletely metabolized the keto acid with transient accumulation of the metabolites mentioned above. The difference between the metabolism byE. coliof hydroxy acid and keto acid is that 12-hydroxyoctadecanoic acid is degraded as far as 6-hydroxydodecanoic acid, while 12-ketooctadecanoic acid can be degraded as far as 4-ketodecanoic acid.