In this paper two major morphological defects that occur in the growth of GaAs and AlxGa1−xAs by molecular beam epitaxy (MBE) are discussed. A uniform, fine scale (<1000 Å period) roughness found on AlxGa1−xAs wafers grown at certain temperatures is shown to result from the presence of a thin Ga layer that has segregated on the surface during growth. The surface roughness leads to high interfacial recombination velocities at GaAs–AlxGa1−xAs heterojunctions and strongly degrades heterostructure device performance, especially lasers. Larger (<20 μm) but localized (density ∼103–105cm−2) oval defects have been proposed to result from spitting of globules of material from the group III element effusion cells. Here, evidence will be presented in support of this hypothesis. The shape of these oval defects as well as the AlxGa1−xAs surface roughness are shown to be related to the anisotropy of the interfacial energy of the group III elements on GaAs and AlxGa1−xAs.