The gene of b-glucuronidase from Thermotoga maritima was cloned into the plasmid pHsh, and expressed in Escherichia coli JM109. The recombinant protein was purified to homogeneity by a simple step, heat treatment. The recombinant enzyme had a molecular mass of 65.9 kD. The optimal activity of b-glucuronidase was found at pH 5.0 and 80oC. The purified enzyme was stable over a pH range from 5.8 to 8.2 and had a half life of 2 h at 80oC. The kinetic experiments at 80oC with p-nitrophenyl-b- glucuronide as substrate gave a Km and Vmax of 0.18 mmol/L and 312 u per mg of protein. The purified enzyme could transform glycyrrhizin to glycyrrhetinic acid.