Homogeneous (as judged by data from gel filtration, ultracentrifugation, polyacrylamide gel electrophoresis with and without sodium dodecyl sulfate (SDS), and isoelectric focusing) beta-xylosidase showing beta-D-xylosidase, beta-D-glucosidase, beta-D-galactosidase, and alpha-L-arabinosidase activities has been isolated from the hemicellulase preparation of the microscopic fungus Aspergillus niger 15 by ethanol fractionation and chromatography on Sephadex G-50, cellulose DE-52, and Sephadexes SP C-50 and G-200. The specific activity of the enzyme toward p-nitrophenyl-beta-D-xylopyranoside (p-NPX) increased 199-fold and was equal to 35.2 units/mg of protein; the activity yield was 43%. The sedimentation coefficient was equal to 10.6 S, and the molecular weight was 253,000 according to the gel filtration data and 122,000 according to the data from SDS electrophoresis. The isoelectric point was at pH 4.9. An amino acid analysis has shown that dicarboxylic and hydrophobic amino acids prevail in the enzyme. beta-Xylosidase had no carbohydrate component, and p-chloromercuribenzoate inhibited its activity. The temperature optimum of beta-xylosidase activity toward p-NPX was at 70 degrees C, and the pH optimum was 3.8-4.0. The enzyme was stable at pH 3 to 8 and did not lose its activity for 1 h at temperatures up to 50 degrees C. D-Xylose was found to be a competitive inhibitor of the beta-D-xylosidase activity of the enzyme with Ki = 2.9 mM. beta-Xylosidase showed transglycosylase activity.