Transmission electron microscopy (TEM) is used to analyse the atomic structure and morphology of misfit dislocations, stacking faults and microtwins at the (001) GaAs on Si interface. Common structural features of GaAs grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) are discussed rather than a series of samples grown under specific growth conditions. In addition to the two well-characterized misfit dislocation types with 1 2a〈110〉 Burgers vectors, part ial dislocations are shown to play a significant role in the mismatch relaxation process terminating stacking faults and microtwins. © 1990.