Two types of laboratory-scale bioreactors were designed for H-2 production by purple nonsulfur bacteria. The bioreactors employed a unique type of hydrogenase activity found in some photosynthetic bacteria that functions in darkness to shift CO (and H2O) into H-2 (and CO2). The mass transport of gaseous CO into an aqueous bacterial suspension was the rate-limiting step and the main challenge for bioreactor design. Hollow-fiber and bubble-train bioreactors employing immobilized and free-living bacteria have proven effective for enhancing the mass transfer of CO. The hollow-fiber bioreactor was designed so that both a growth medium and CO (10% in N-2) passed from the inside of the fibers to the outside within the bioreactor. Bacteria were immobilized on the outer surface of the hollow fibers. Hydrogen production from CO at an average rate of 125 ml g cdw(-1) h(-1) (maximum rate of 700 ml g cdw(-1) h(-1)) was observed for more than 8 months. The bubble-train bioreactor was built using polyvinyl chloride (PVC) tubing, wound helically on a vertical cylindrical supporting structure. Small bubbles containing CO were injected continuously through a needle/septum connection from the gas reservoir (20% CO). Up to 140 ml g cdw(-1)h(-1)of H-2 production activity was observed using this bioreactor for more than 10 days.