The morphology of interfacial dislocations in molecular‐beam epitaxy (MBE) grown GaAs/InxGa1−xAs/GaAs thin films (60–300 nm,x=0.15–0.40) on GaAs(100) undergoes a transition with increasingxfrom rectangular arrays, with dislocations lying in the 〈011〉 directions, to random tangled arrays, with a reduced preference for crystallographic orientation. Films with intermediatexvalues exhibit both types of morphology in separate areas. For a givenxvalue the interfacial dislocation density increases with increasing thickness. A marked asymmetry in the dislocation density in the [01̄1] and [011]directions is observed in some of the films exhibiting rectangular arrays for specific values of InxGa1−xAs film thickness above the critical value. The asymmetry is not observed in thicker InxGa1−xAs films. This asymmetry is attributed to different mobility of α and β dislocations. The rectangular array morphology is found in a number of lattice‐mismatched systems, including early observations in GaAsxP1−x/GaAs, while the random arrays are representative of recent observations in GaAs/Si. The measured dislocation densities in the InxGa1−xAs /GaAs system are generally inadequate to relax the interfacial strain, implying a significant partitioning of strain between elastic and plastic components. In 300 nm thick films, a dislocation pinning reaction was directly observed. This reaction is most probably responsible for (1) creating dislocation sources that facilitate strain relaxation in the early stages of strain relaxation, and (2) inhibiting dislocation motion in the latter stages of strain relaxation by pinning dislocations, thus producing a high density of threading dislocations. These observations were enabled by a new technique for preparing large‐area electron transparent films, which required no thinning by ion milling or any other method.