Fuel cells with a Pseudomonas methanica suspension at the anode and sterile medium at the cathode developed 0.5–0.6v on open circuit, methane and air being used. Maximum power density was 2.8 µw/cm2 (2.6 mw/ft2) at 0.35v. Hydroxyl ions are the source of oxygen at the anode. The immediate fuel is hydrogen, probably supplied by a flavoprotein. The free energy efficiency of the fuel cell is 14.2% at very low drain and 8.2% at the maximum power point. Its improvement depends on finding energetically more advantageous ways of interfering electrochemically in the natural respiratory sequence. Current density can be improved, possibly by one order of magnitude.