Hydrogen (H-2) production from simulated cheese processing wastewater via anaerobic fermentation was conducted using mixed microbial communities under mesophilic conditions. In batch H-2 fermentation experiments H-2 yields of 8 and 10mM/g COD fed were achieved at food-to-microorganism (F/M) ratios of 1.0 and 1.5, respectively. Butyric, acetic, propionic, and valeric acids were the major volatile fatty acids (VFA) produced in the fermentation process. Continuous H-2 fermentation experiments were also performed using a completely mixed reactor (CSTR). The pH of the bioreactor was controlled in a range of 4.0-5.0 by addition of carbonate in the feed material. Maximum H-2 yields were between 1.8 and 2.3 mM/g COD fed for the loading rates (LRs) tested with a hydraulic retention time (HRT) of 24 It. Occasionally CH4 was produced in the biogas with concurrent reductions in H2 production; however, continuous H-2 production was achieved for over 3 weeks at each LR. The 16S rDNA analysis of DNA extracted from the bioreactors during periods of high H-2 production revealed that more than 50% of the bacteria present were members of the genus Lactobacillus and about 5% were Clostridia. When H-2 production in the bioreactors decreased concurrent reductions in the genus Lactobacillus were also observed. Therefore, the microbial populations in the bioreactors were closely related to the conditions and performance of the bioreactors. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.