Using cross‐section and plan‐view transmission electron microscopy we demonstrate that the initial plastic relaxation of highly mismatched layers grown along [100] is governed by the growth mode. During MBE growth of InAs on GaAs in the Stranski‐Krastanov (SK) mode, 60o ‐type dislocations are generated at the island edges and then glide to the interface to relieve the strain. The resulting interfacial microstructure consists of an inefficient arrangement of misfit dislocations. On the other hand, when InAs is forced to grow in a two‐dimensional (2D) mode, only pure edge‐type dislocations are generated and they are located exactly at the epilayer/substrate interfacial plane. These results are explained by a different dislocation nucleation mechanism imposed by the planar morphology of the highly strained film. ©1995 American Institute of Physics.