A new bacterium, Citrobacter sp. Y19, catalyzing the water-gas shift reaction was isolated from an anaerobic wastewater sludge digester. It grew aerobically with a high specific growth rate of 0.7 h(-1) in a mineral salt medium supplemented with yeast extract and Bacto-tryptone, and produced H-2 under anaerobic conditions after the cells were transferred to tryptone-deleted medium. The maximum H-2 production rate was 33 mmol H-2 g(-1) cell h, which was maintained for about 200 h. This is the first report on a chemoheterotrophic bacterium which utilizes CO with the production of H-2 and CO2.