A very active and specific enzyme, aldehyde oxidase, has been demonstrated and partially purified from the digestive tract of the marine borer, Bankia setacea (Tryon). In the presence of the enzyme, the substrate vanillin was oxidized to vanillic acid with the simultaneous reduction of oxygen, cytochrome c, 2,6-dichlorophenolindophenol, or methylene blue. Marine borer aldehyde oxidase does not oxidize the heterocyclic substrates N1-methylnicotinamide or hypoxanthine of mammalian hepatic aldehyde oxidase or milk xanthine oxidase. In addition, the inhibitors of hepatic aldehyde oxidase, menadione and estradiol, do not inhibit marine borer aldehyde oxidase. On the other hand, both the above-mentioned oxidases are inhibited by the sulfhydryl inhibitors arsenite and p-hydroxymercurybenzoate, and by cyanide. Marine borer aldehyde oxidase has an optimum activity between pH 4.5 and 5.5 and little activity at pH 7.8, a pH activity range quite different from that of the mammalian aldehyde oxidases. Oysters were found to contain only slight vanillin oxidase activity at pH 4.3, suggesting that the marine borer aldehyde oxidase activity is unique to the ship worm and is used for the oxidation of aldehydes found in wood.