Ribulose 1,5-bisphosphate carboxylase/oxygenase was purified from a thermophilic hydrogen-oxidizing bacterium, Pseudomonas hydrogenothermophila strain TH-1. The enzyme was an L(8)S(8)-type hexadecamer with a molecular mass of 560 kDa. The specific activity of the purified enzyme was about 6.5 mu mol/mg and stable even by heating at 60 degrees C for 30 min. The K-m values for RuBP, CO2, and Mg++ were 1.03 mu M, 0.086 mM and 0.92 mM, respectively.