Direct electrochemical response of cytochrome c (Cyt c), which was adsorbed onto the surface of multiwalled carbon nanotubes (MWNTs) through a simple cyclic potential scan technique, was observed. Cyt c adsorbed on MWNTs showed an enzyme-like activity to catalyze the reduction of oxygen (O-2). Thus, a third generation O-2 reagentless biosensor was constructed.