A recombinant putative beta-galactosidase from Thermoplasma acidophilum was purified as a single 57 kDa band of 82 U mg(-1). The molecular mass of the native enzyme was 114 kDa as a dimer. Maximum activity was observed at pH 6.0 and 90A degrees C. The enzyme was unstable below pH 6.0: at pH 6 its half-life at 75A degrees C was 28 days but at pH 4.5 was only 13 h. Catalytic efficiencies decreased as p-nitrophenyl(pNP)-beta-d-fucopyranoside (1067) > pNP-beta-d-glucopyranoside (381) > pNP-beta-d-galactopyranoside (18) > pNP-beta-d-mannopyranoside (11 s(-1) mM(-1)), indicating that the enzyme was a beta-glycosidase.