Single-crystal electron-diffraction patterns of synthetic chlorapatite [Ca5(PO4)3Cl] show weak reflections violating the space group P21/b. Transmission electron microscopy with dark-field imaging, using hk0 (k = even) reflections, which are allowed for P21/b, reveals no special features, while dark-field viewing through certain superstructure reflections [hk0 (k = odd), which are forbidden for this space group] reveals antiphase domains. Diffraction-contrast experiments show that these domains are related by a displacement vector R = ½[010]. This displacement vector explains the origin of the domain in terms of the ordering of the Cl-ion position in the channels of chlorapatite.