Immunocytochemical studies have localized the glucose oxidase in Aspergillus niger in the cell wall. In addition, non-denaturating polyacrylamide gel electrophoresis has established the presence of four distinct catalases. Two are constitutive, while the other two are inducible enzymes. These catalases are postulated to play a major role in the protection of glucose oxidase by degrading hydrogen peroxide formed in the cell wall. Theyalso appear to be involved in intracellular detoxification but data is still inconclusive.