A β-d-galactosidase exhibiting high activity in the alkaline pH region was purified from Teratosphaeria acidotherma AIU BGA-1, which we previously isolated as a unique fungal producer of three acidophilic and one alkalophilic β-d-galactosidases (Isobe etal., J. Biosci. Bioeng., 116, 171-174, 2013). The enzyme was stable in the pH range 7.5-10.0 and exhibited optimal activity at pH 8.0 and 60°C. The enzyme hydrolyzed 2-nitrophenyl β-d-galactopyranoside, 4-nitrophenyl β-d-galactopyranoside, and lactose, and the Km values were estimated to be 0.349mM, 0.488mM, and 701mM, respectively. Chelating reagents (EDTA and o-phenanthroline) and metals (Cu2+and Ni2+) inhibited the enzyme activity, and Mn2+ was a good activator. The enzyme also exhibited transgalactosylation activity for lactose. The enzyme's molecular mass was estimated to be 180kDa, and its structure was monomeric. Thus, the enzymatic and physicochemical characteristics of the alkalophilic β-galactosidase in this study clearly differed from those of the previously known alkalophilic β-d-galactosidases.