Abstract IN our strain of Acetobacter suboxydans, three enzymes are present for the oxidation of gluconate: 1. (1) a soluble TPN-specific dehydrogenase, yielding 2-ketogluconate (2-ketogluconoreductase); 2. (2) a soluble TPN-specific dehydrogenase, yielding 5-ketogluconate (5-ketogluconoreductase); 3. (3) a particulate, possibly cytochrome-linked gluconate oxidase, yielding 2-ketogluconate. The specificity of these enzymes was studied. Galactonate is oxidized with a TPN-linked dehydrogenase to 2-ketogalactonate. Soluble TPN-linked dehydrogenases for glucose, mannose, galactose, xylose and l -arabinose are present. A 5-ketogluconokinase was not detectable. Both 2- and 5-ketogluconates are metabolized by a reductase into gluconate, followed by a gluconokinase and a 6-phosphogluconodehydrogenase. A strain of Acetobacter melanogenum was also investigated because in a period of one year it had lost the power to make a brown pigment and now produced considerable amounts of Ca-5-ketogluconate. The new behaviour of this strain was due to the fact that the 2-ketogluconoreductase was absent and the gluconate oxidase content was low. 5-Ketogluconate is not metabolized by this strain, owing to the absence of a gluconokinase.