The beta-galactosidase exhibiting high activity from an extremely acidic pH region to neutral pH region was efficiently purified from an acidophilic fungus, Teratosphaeria acidotherma AIU BGA-1, using affinity chromatography with Toyopearl resins immobilized 4-aminophenyl-beta-D-galactopyranoside. The enzyme was stable in the pH range from 1.5 to 7.0, and exhibited optimal activity at pH 2.5-4.0 and 70 degrees C. 2-Niteophenyl-beta-D-galactopyranoside, 4-nitrophenyl-beta-D-galactopyranoside and lactose were rapidly hydrolyzed, and the apparent Km values were estimated to be 0.19 mM, 1.2 mM and 170 mM, respectively. Thus, the enzyme can be used in the wide pH range for hydrolysis of lactose. The molecular mass of the enzyme was estimated to be 140 kDa with two hetero subunits of 86 kDa and 50 kDa. The N-terminal amino acid sequence of the small subunit was found to be NTRMIIFNDK. These enzymatic and physicochemical characteristics are remarkably different from those of the previously known beta-galactosidases. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.