The effect of cathodic polarization on anisotropic pitting behavior of aluminum in 1M hydrochloric acid solution was investigated using single crystals with different surface orientations. The pit density on aluminum single crystals was markedly increased by the presence of cathodic polarization, regardless of their surface orientations. A trend towards anisotropic pitting behavior was exhibited such that the pit density increased in the order (110) less than or equal to (100) < (111). It follows from the calculated pit current densities that the (110) plane most rapidly dissolves.