The objective of this study was to improve the biological water-gas shift reaction for producing hydrogen (H-2) by conversion of carbon monoxide (CO) using an anaerobic thermophilic pure strain, Carboxydothermus hydrogenoformans. Specific hydrogen production rates and yields were investigated at initial biomass densities varying from 5 to 20 mg volatile suspended solid (VSS) L-1. Results showed that the gas-liquid mass transfer limits the CO conversion rate at high biomass concentrations. At 100-rpm agitation and at CO partial pressure of 1 atm, the optimal substrate/biomass ratio must exceed 5 mol CO g(-1) biomass VSS in order to avoid gas-liquid substrate transfer limitation. An average H-2 yield of 94 +/- 3% and a specific hydrogen production rate of ca. 3 mol g(-1) VSS day(-1) were obtained at initial biomass densities between 5 and 8 mg VSS-1. In addition, CO bioconversion kinetics was assessed at CO partial pressure from 0.16 to 2 atm, corresponding to a dissolved CO concentration at 70A degrees C from 0.09 to 1.1 mM. Specific bioactivity was maximal at 3.5 mol CO g(-1) VSS day(-1) for a dissolved CO concentration of 0.55 mM in the culture. This optimal concentration is higher than with most other hydrogenogenic carboxydotrophic species.