The surface of a Co(0001) single crystal electrode was examined by in situ electrochemical scanning tunneling microscopy (STM) in 0.05 M Na2SO4 at pH 3 under potential control. Cyclic voltammetry indicated that the anodic dissolution of Co proceeds without passivation, and the anodic current was almost proportional to applied potential. The Co(0001)-(1 X 1) structure was observed on atomically flat terraces in the potential range between the hydrogen evolution and the onset of the anodic dissolution. At the foot of the rising anodic current-potential curve, a hexagonal surface lattice with (5 X 5) symmetry corresponding closely to that of CoO(111) or Co(OH)(2)(0001) was also observed. In the early stages of anodic dissolution, the process proceeded in the layer-by-layer mode, forming atomically flat terraces extending over large areas. By reapplying a negative potential after the anodic dissolution, well-defined terrace-step structures were restored, suggesting that the electrodeposition also took place preferentially at step edges.