The electro-kinetic properties of Arthrobacter siderocapsulatus were determined using microelectrophoresis in alternating electric field. The surface of bacterial cells was shown to bear positive charge within a wide range of pH. The positive charge of the cells should be attributed apparently to the presence of mucous capsules containing manganese dioxide since the growth of the bacterium is known to involve oxidation of manganese or ferrous iron and accumulation of their hydroxides in the capsules. Because of a high respiration rate and the presence of mucous capsules or sheaths, the cells settling on the surface of metals create zones of differential aeration, thus stimulating corrosion. Studies of the electro-kinetic properties of iron bacteria make it possible to develop protective measures against corrosion and are therefore of practical significance.