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　Enzyme-immobilized electrodes were fabricated with the conducting films of a polythiophene derivative having carboxyl groups.　The conducting films were prepared by electrochemical copolymerization of 3-methylthiophene and thiophene-3-acetic acid, and redox enzymes were immobilized covalently by the condensation reaction with the carboxyl groups on the films.　The electrochemical properties of the enzyme-immobilized electrodes were investigated in connection with the polymerization condition.　The glucose oxidase-immobilized electrode showed a significant oxidation current in the presence of glucose.　The oxidation current was found to increase with an increasing amount of the charge passed through during the electrochemical polymerization.　This result was attributed to an increase in the surface area of the polymer film due to the formation of a fibrous structure, which was considered to facilitate the bioelectrochemical reaction on the electrode.　The glucose oxidase-immobilized anode and the bilirubin oxidase-immobilized cathode were applied to a glucose / oxygen fuel cell, and the power output of 0.35 mW/cm² was obtained.