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The redox balance in microbial metabolism is an important factor to determine end-products in the microbial fermentation. We supplied reducing power to a Clostridium strain to manipulate the redox metabolism using a cathode as a bioelectrochemical technique (1-3). The cyclic voltamogram showed a redox peak at -160 mV v.s. Ag/AgCl in the Clostridium culture grown in P2 medium with glucose as a carbon source. Electrochemically active Clostridium sp. was cultivated in a cathode compartment at an applied potential of -160 mV vs. Ag/AgCl. The electric current was consumed by the Clostridium without any mediator and it was increased to 1.5 mA according to the microbial growth. Interestingly, the production of butyrate and acetate from glucose was decreased in this bioelectrochemical system. However, butanol production (4 NADH consumed for 1 butanol) was increased up to 2.4 times. We measured the ratio of NADH/NAD+ within cells and found it was dependent on the current change. It was supposed that the cathode reduction of NAD+ induced more butanol production. This is the first report of controlling the cellular metabolism of the electroactive Clostridium given by an electrochemically introduced reducing equivalent.