A methane-producing biocathode that converts CO(2) into methane was studied electrochemically and microbiologically. The biocathode produced methane at a maximum rate of 5.1 L CH(4)/m(2) projected cathode per day (1.6 A/m(2)) at -0.7 V versus NHE cathode potential and 3.0 L CH(4)/m(2) projected cathode per day (0.9 A/m(2)) at -0.6 V versus NHE cathode potential. The microbial community at the biocathode was dominated by three phylotypes of Archaea and six phylotypes of bacteria. The Archaeal phylotypes were most closely related to Methanobacterium palustre and Methanobacterium aarhusense. Besides methanogenic Archaea, bacteria seemed to be associated with methane production, producing hydrogen as an intermediate. Biomass density varied greatly with part of the carbon electrode covered with a dense biofilm, while only clusters of cells were found on other parts. Based on our results, we discuss how inoculum enrichment and changing operational conditions may help to increase biomass density and to select for microorganisms that produce methane.