We report unipolar resistive switching measurements from single vertical ZnO nanowires grown directly on a copper substrate. Electrical measurements using a conductive atomic force microscope show conductive filament formation at 30 kV/cm, which is an order of magnitude lower field than that for bulk films and suggest a preferential filament formation likely on the surface of the nanowires. A high resistive ratio of three orders of magnitude was observed between the high and low resistive memory states. (C) 2013 Elsevier B. V. All rights reserved.