Abstract Ascorbic acid (AA) is oxidized by two soluble enzyme systems in spores and mycelium of the fungus, Myrothecium verrucaria. One system, for which the name AA Oxygenase is suggested, catalyzes the reaction: AA + O2 → oxalic acid + threonic acid. Such a system has not been reported before. The enzyme appears to be specific for AA. Ferrous ions, which are weakly bound to the protein, are required for activity. Nevertheless, cyanide and EDTA are not inhibitory. The other system, which has been termed “atypical” AA oxidase because it is not markedly inhibited by cyanide, catalyzes the reaction: AA + 1 2 O2 → dehydroascorbic acid + water. The enzyme oxidizes AA and several of its close analogs and requires O2 as an electron acceptor. It is not inhibited by metal chelators, sulfhydryl reagents, or flavin-enzyme inactivators and does not lose activity on dialysis against cyanide or EDTA. In the presence of AA + O2, the oxidase is rapidly inactivated by high concentrations (5.6 × 10−3M) of H2O2 or by Fenton's reagent (Fe++ + H2O2). Enzyme activity is progressively lost during the oxidation of AA. The loss, termed “reaction inactivation,” is not due to inhibition by reaction products or to formation of free H2O2. Hydrogen peroxide at low concentrations (5.6 × 10−4M) stimulates oxidase activity.