A newly isolated Citrobacter sp. Y19 grows on organic carbons aerobically and produces hydrogen from carbon monoxide (CO) and water when transferred to anaerobic conditions. Hydrogen production capability of Y19 was studied in serum-bottle and bioreactor cultures. Optimal cell growth was observed at pH 5-8, temperature of 30-40degreesC, oxygen partial pressure of 0.2-0.4 atm. Induction of hydrogen production activity could be carried out efficiently under 20% (v/v) CO when the culture was removed to anaerobic conditions at 12 h. Optimal conditions for hydrogen production were 30-40degreesC and pH 5.5 -7.5. The maximum hydrogen production activity was observed as 27.1 mmol/g cell h, which was about three times higher than that of Rhodospirillum rubrum. In bioreactor experiments, a stable hydrogen production along with a high activity of 20 mmol H-2/g cell h was observed during the continuous operation of 68 h. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.