CueO from Escherichia coli, a member of the multi-copper oxidase (MCO) family, was examined as a direct electron transfer-type bioelectrocatalyst for the four-electron reduction Of O-2. Although CueO requires the fifth copper located near the type I Cu site to exhibit its oxidase activity (Roberts et al., 2003), it has been found that CueO receives electrons directly from electrodes even in the absence of the fifth copper. The fact indicates that electrons are transferred directly from electrodes to the type I Cu site. Furthermore, the catalytic current density of as large as about -4 mA cm(-2) was successfully observed with a rotating pyrolytic graphite electrode at pH 5. CueO is found to be superior to other MCOs in view of the catalytic activity and is an important candidate as a catalyst of the cathode in biofuel cells.