Hydrogen gas was produced using granular sludge of mixed culture from sewage digester sludge. Methane gas was detected in the biogas even under acidogenic condition with pH 5.5-6.2 and after heat treatment at 95 degrees C. In order to repress methane production, nitrate known to inhibit methanogenic bacteria was added to an anaerobic reactor. When the feeding concentration of KNO3 into the reactor was 1000 mg/L and more, the yield of hydrogen gas production (H-2-mol/glucose-mol) was almost 1.0 and no methane gas was produced. The nitrate was thought to repress methanogenic bacteria remarkably and this was confirmed using a SEM. (c) 2005 Elsevier Ltd. All rights reserved.