A beta-xylosidase was purified from the culture filtrate of the thermophilic fungus Thermoascus sp. by ultrafiltration, ethanol precipitation, and chromatography with DEAE-Toyopearl 650M, Mono Q HR5/5, and Phenyl Superose HR5/5. The purified beta-xylosidase was found to be homogeneous on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Its molecular weight was estimated to be 107 kDa by gel filtration chromatography (Superdex 200 HR) and 100 kDa by SDS-PAGE. The optimum activity of the enzyme was observed at pH 4.5 and 55 degrees C. The enzyme was stable up to 60 degrees C at pH 4.5 for 1 h. The enzyme exhibited hydrolytic activity on phenyl beta-D-xyloside and xylan (birch wood). Fifteen of the amino acid residues in the amino terminal region of the Thermoascus sp. beta-xylosidase were homologous with residues in the equivalent region of the maturation protein beta-glucosidase from Aspergillus aculeatus.