Nanowires consisting of regioregular poly(3-hexylthiophene) (P3HT) were prepared by whisker precipitation from a solution of P3HT in anisole. Photoirradiation of two electrodes modified with P3HT nanowires or P3HT with visible light afforded cathodic photocurrents in the presence of O(2). The photocurrent of the electrode modified with P3HT nanowires was approximately twice as large as that from the electrode modified with P3HT. This is most likely because hole hopping between adjacent thiophenes is facilitated in the P3HT nanowires due to pi-pi stacking.