After depositing a 1/6 monolayer of AlAs on a very flat GaAs (001) surface by metalorganic vapor‐phase epitaxy, we have studied AlAs two‐dimensional (2D) nuclei by high‐vacuum scanning tunneling microscopy. AlAs 2D nuclei elongate in the [110] direction, like GaAs. The density of AlAs 2D nuclei in the saturation region was 5×1010cm−2at 580 °C. The saturated AlAs 2D nucleus density decreased as the temperature increased. From the saturated AlAs 2D nucleus densities the surface diffusion coefficient of AlAs on GaAs was calculated to be 1.5×10−7cm2/s at 530 °C. This is one order of magnitude smaller than that of GaAs on GaAs. ©1995 American Institute of Physics.