Methane fermentation was successfully carried out in bioelectrochemical reactors without membranes under a working potential of -0.6 or -0.8 V (vs. Ag/AgCl) and neutral pH conditions. The hydrogenotrophic methanogens that dominated on the anodic and cathodic electrodes differed from those found on the electrodes in the control reactors without electrochemical reactions. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.