An aciduric facultative anaerobe with a hydrogen-producing ability was isolated and identified as Enterobacter aerogenes strain HO-39. The bacterium was able to grow at acidic pH of 3.3 aerobically and at 4.0 anaerobically. Although the optimum pH for hydrogen production was 6.0 to 7.0, hydrogen could be produced at acidic pH of 4.0. The optimum temperature for hydrogen production and cell growth was 38 degrees C. Among various carbohydrates, glucose was suited for hydrogen production and the yield was 1.0 mol-H-2/mol-glucose. Polypepton was a favorable nitrogen source and hydrogen production was suppressed in a medium where glucose and Polypepton were autoclaved simultaneously. In a continuous culture, stable hydrogen production without the need for pH control in a reactor was attained under the condition where pH of the culture broth was more than 4.5.