In order to study the enhancement of crevice corrosion due to fouling of sea organisms on stainless steel surfaces, a series of corrosion tests were performed using stainless steel tanks as specimens. During each test live seawater was over-flowed so that the sea organisms should foul on the surface of the tanks. The results obtained were as follows: 1) In calm seawater, the corrosion potential of the stainless steel was found to be ca. 100mV (vs. SCE) for free surface, to increase up to ca. 200mV as being fouled by brown slime, and to exceed 300mV as being fouled by algae, barnacles, mussels, and other organisms. 2) The corrosion potential of the stainless steel in this environment appeared to be controlled mainly by the amount of dissolved oxygen in seawater. 3) The presence of fairly large area of cathode was necessary for the steady propagation of crevice corrosion.