An isolated pit and connected pits with one-monolayer depth have characteristic step-edge shapes on a GaAs (110) surface fabricated by a cleaved-edge overgrowth (CEO) method and high temperature growth-interrupt annealing. First principles calculations of Ga and As adatoms have shown that both Ga and As adatoms are most stable at the sites near As of the topmost layer while they are unstable at those near Ga, which is expected to be the microscopic origin for the formation of the fish-shaped pits. (C) 2004 Elsevier B.V. All rights reserved.