We report the first single-chamber microbial electrochemical cell for conversion of CO2 to CH4, with an average CH4 production rate of 0.47 +/- 0.05mL day(-1)cm(-2) at an applied potential of 600mV, utilizing a methanogenic microbial community collected from the formation water in the San Juan coal basin (Colorado, USA). CH4 production was only observed at the graphite rod cathode after an electrochemical pre-treatment that facilitates biofilm formation. The carbon contained within the CH4 arose predominantly from the CO2 source, as verified by experiments during which the CO2 source was repeatedly turned off and on. Modest quantities of acetic acid and ethanol were also produced. DNA extraction and sequencing from the microbial community showed that from the Archaea kingdom, only 2 species survived prolonged exposure to CO2 and CH4 production, methanobacterium sp. (81.4%), and methanoculleus sp. (18.6%), while in the bacterial kingdom, anaerobaculum thermoterrnum (67.1%) was the predominant surviving species.