The effect of gas sparging on continuous fermentative H(2) production was investigated in completely stirred-tank reactors (CSTR) using internal biogas, N(2) and CO(2) with various flow rates (100, 200, 300 and 400 ml/min). The sparging with external gases of N(2) and CO(2) showed higher H(2) yield than the control of no sparging and internal biogas sparging. It indicated that the decrease of H(2) partial pressure by external gas sparging had a beneficial effect on H(2) fermentation. Especially, CO(2) sparging was more effective in the reactor performance than N(2) sparging, accompanied by higher production of H(2) and butyrate. The best performance was obtained by CO(2) sparging at 300 ml/min, resulting in the highest H(2) yield of 1.68 mol H(2)/mol hexose(consumed) and the maximum specific H(2) production rate of 6.89 L H(2)/g VSS/day. Compared to N(2) sparging, there might be another positive effect in CO(2) sparging apart from lowering H(2) partial pressure. High CO(2) partial pressure had little effect on H(2)-produrcing bacteria but inhibitory effect on other microorganisms such as acetogens and lactic acid bacteria which were competitive with H(2)-producing bacteria. Only H(2)-producng bacteria, such as Clostridium tyrobutyricum, C. proteolyticum and C. acidisoli were isolated under CO(2) sparging conditions based on 16S rDNA analysis by PCR-DGGE. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.