Direct electron transfer between laccase and a glassy carbon electrode modified with carbon nanotubes having a uniform inner tube diameter was observed by cyclic voltammetry in 0.10 M phosphate buffer. The formal potential of +530 mV (vs. SCE) was very close to redox potential of T1 copper in laccase. No direct electron transfer between laccase and a glassy carbon electrode modified with carbon nanotubes having a tapered inner tube diameter was determined under the same condition. The possible application of the laccase-catalyzed O-2 reduction at these electrodes was successfully illustrated by constructing an ascorbate/O-2 biofuel cell. (c) 2008 Elsevier B. V. All rights reserved.