We report the results of an investigation of the structural properties and relaxation of misfit stress with transmission electron microscopy and x‐ray diffraction techniques. Epitaxial films of ZnSe were grown on GaAs by molecular‐beam epitaxy of thicknesses ranging from 0.05 to 4.9 &mgr;m. The films contain stacking fault defects up to thicknesses of about 150 nm. Above 150 nm perfect misfit dislocations are generated from surface sources and the stacking fault defects to accommodate the lattice mismatch. The majority of dislocations observed are of the 60° type with Lomer edge type dislocations observed in a much lower concentration. The density of misfit dislocations increases with increasing epilayer thickness. Above about 1 &mgr;m the films exhibit biaxial tension which we believe is due to thermal expansion differences of ZnSe and GaAs. Good agreement is observed between microscopic and diffraction measurements of the relaxation phenomena.