Observations of antiphase disorder obtained using a new technique of transmission electron microscopy, in (100) epitaxial layers of GaAs:Ge produced by MBE are presented. The crystallographic origin of this type of disorder is analysed using a recently developed approach. It is shown that antiphase disorder can exist in epitaxial layers where the substrate orientation is of the form {hko}this is consistent with experimental observations that disorder is observed on (100) and (110) substrates, but not on (111) or (211), for example. Antiphase boundaries in {hko} specimens are shown to separate interfacial domains which are energetically degenerate and related by symmetry operations.