Previously unreported two‐dimensional lattice mismatch in planes parallel to the interfacial boundary between the (001) Ga0.65Al0.35As epitaxial layer and the GaAs substrate has been observed by utilizing a four‐beam, (000), (400), (220), and (22¯0), simultaneous Borrmann diffraction of x rays. The shifts of the reflection bands of the layer from the reflection lines of the substrate indicate that shear stresses exist in ⟨100⟩ and ⟨010⟩ directions and that the corresponding strains vary continuously along the thickness direction of the epilayer.