A beta-galactosidase exhibiting maximal activity at pH 1.0 was purified from Teratosphaeria acidotherma AIU BGA-1. The enzyme had a molecular mass of 180 kDa and consisted of two heterosubunits of 120 kDa and 66 kDa. The N-terminal amino acid sequence of the large subunit was found to be SPNLQDIVTVDGESY. These physicochemical properties differed from those of other microbial beta-galactosidases. At pH values of 1.5 and pH 4.5, the enzyme exhibited its highest activity at temperatures of 70 degrees C and 80 degrees C, respectively. Thus, the enzyme exhibited the lowest optimal pH and highest optimal temperature among the microbial beta-galactosidases thus reported. The enzyme retained more than 80% of its original activity in the pH range from 2.0 to 8.0 by incubation at 50 degrees C for 30 min. The enzyme hydrolyzed 4-nitrophenyl-beta-D-fucopyranoside, 2-nitrophenyl-beta-D-galactopyranoside, and 4-nitrophenyl-beta-D-galacto-pyranoside at relative reaction rates of 100, 59, and 24, respectively, at pH 1.5, and its affinity for beta-D-galactopyranosides was higher than that for beta-D-fucopyranosides. The enzyme also efficiently hydrolyzed lactose in milk and whey from yoghurt at pH 1.5. (c) 2015, The Society for Biotechnology, japan. All rights reserved.