Catalase from Aspergillus niger was purified to homogeneity as judged from the results of ultracentrifugation and polyacrylamide gel electrophoresis. The enzyme had a molecular weight of 385,000 as estimated from sedimentation measurements. Carbohydrate analyses showed that the catalase was a glycoprotein containing about 8.3% neutral sugar and 1.9% glucosamine. Under denaturing conditions, polyacrylamide gel electrophoresis revealed only one band with a molecular weight of 97,000 daltons in gels stained for either protein or sugar, suggesting that the native enzyme consists of four subunits with covalently bound carbohydrate. In the reaction with inhibitors, A. niger catalase showed lower affinity than the "standard" catalases. The pK values for HCN, HN3, and HF were estimated to be 3.4 (at pH 7.4), 2.3, and 1.5 (at pH 4.2), respectively. In addition, the fungal enzyme reacts with methyl hydrogen peroxide in a very unusual way. Even after the addition of a large excess of the peroxide, only catalase compound I was formed, and compound II did not appear. Using this unique property of A. niger catalase, we obtained CD and MCD spectra of compound I uncontaminated by compound II. The magnitude of the positive CD peak of compound I in the Soret region was about half that of the native enzyme. The MCD spectrum obtained was better resolved than that of bovine liver catalase compound I in the visible region.