A newly isolated Citrobacter sp. Y19 for CO-dependent H-2 production was studied for its capability of fermentative H-2 production in batch cultivation. When glucose was used as carbon source, the PH of the culture medium significantly decreased as fermentation proceeded and H2 production was seriously inhibited. The use of fortified phosphate at 60-180 mM alleviated this inhibition. By increasing culture temperatures (25-36degreesC), faster cell growth and higher initial H2 production rates were observed but final H2 production and yield were almost constant irrespective of temperature. Optimal specific H2 production activity was observed at 36degreesC and pH 6-7. The increase of glucose concentration (1-20 g/1) in the culture medium resulted in higher H2 production, but the yield of H2 production (mol H-2/mol glucose) gradually decreased with increasing glucose concentration. Carbon mass balance showed that, in addition to cell mass, ethanol, acetate and CO2 were the major fermentation products and comprised more than 70% of the carbon consumed. The maximal H2 yield and H2 production rate were estimated to be 2.49 mol H-2/mol glucose and 32.3 mmol H-2/g cell h, respectively. The overall performance of Y19 in fermentative H-2 production is quite similar to that of most H-2-producing bacteria previously studied, especially to that of Rhodopseudomonas palustris P4, and this indicates that the attempt to find an outstanding bacterial strain for fermentative H2 production might be very difficult if not impossible. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.