We have investigated the electrical properties of a fuel cell based on DNA film. It is found that open-circuit voltage appears in the humidity region between 14% and 55% and reaches a maximum at approximately 55%. This result indicates that in the fuel cell based on DNA film, optimum condition exists. From the analyses of the frequency dependence of electrical conductivity, it is also found that electrical conductivity consists of the component of electrical conductivity caused by the dielectric dispersion, and the component of DC electrical conductivity. Furthermore, we have found that the ratio, 6, of DC conductivity to the component caused by dielectric dispersion becomes large in the middle-humidity region between 17% and 60% and reaches a maximum at approximately 57% where open-circuit voltage reaches a maximum. This result indicates that in the middle-humidity region, high-DC conductivity and low-dielectric loss are achieved. From these results, it is deduced that optimum conduction of the fuel cell based on DNA film is determined by not only the proton conductivity but also by the dielectric loss accompanied by the dielectric dispersion. (c) 2005 Elsevier B.V. All rights reserved.