Electromethanogenesis is a new bio-electrochemical reaction potentially useful for energy conversion. As a first step toward its technical application, electromethanogenic reactors were built, and their bio-electrochemical properties were analyzed. Comparisons of the microbial compositions of the electromethanogenic cathode and the current-producing anode suggested an electromethanogenic pathway mediated by exoelectrogenic bacteria. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.