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To investigate the precise effect of the redox potential on the methanogenesis of the hydrogenotrophic methanogen Methanothermobacter thermautotrophicus by using an electrochemical redox controlling system without adding oxidizing or reducing agents. A bioelectrochemical system was applied to control the redox conditions in culture and to measure the methane-producing activity of M. thermautotrophicus at a constant potential from +0·2 to −0·8 V (vs Ag/AgCl). Methane production and growth of M. thermautotrophicus were 1·6 and 3·5 times increased at −0·8 V, compared with control experiments without electrolysis, respectively, while methanogenesis was suppressed between +0·2 and −0·2 V. A clear relationship between an electrochemically regulated redox potential and methanogenesis was revealed.A novel bioelectrochemical method can activate the methanogenesis of M. thermautotrophicus by controlling the redox potential in culture conditions at −0·8 V, which is a difficult potential to achieve by conventional methods (e.g. by adding reducing agents). This study provides useful insights for the application of a bioelectrochemical system in industrial processes involving methanogens, such as in anaerobic digesters.