Electrochemical killing of the marine gram-negative bacterium, Vibrio alginolyticus, attached to a basal-plane pyrolytic graphite electrode modified with ferrocene was carried out. The survival ratio decreased to 43% when a low potential of 0.2 V versus a saturated calomel electrode was applied for 30 min and was 3% after applying 0.4 V. After the potentials were applied, the cells were stained with the fluorescent dye, propidium iodide (PI), and the change in cell membrane permeability was observed. PI-stained cell ratio almost correlated with the dead cell ratio obtained by the colony formation method when using the bare graphite electrode. In contrast, when using the ferrocene-modified electrode, PI-stained cell ratio did not correlate with the dead cell ratio and increased when potentials of above 0.8 V were applied. When using the ferrocene-modified electrode, the killing effect was not due to the change in cell membrane permeability and might be due to the electrocatalytic oxidation of intracellular substances.