Neutron diffraction measurements have been carried out on an Ni3Al0·8Mn0·2alloy at different degrees of cold-working. The Mn atoms occupy the Al sublattice in the L12-type Ni3Al structure. In the plastically deformed state, however, a long-period superstructure with a modulation of the lattice spacing appears. The superstructure can be taken as being composed of antiphase domains, where the alignment direction of the domains is parallel to the ⟨111⟩ axis of the cubic L12-type cell and a step shift occurs in close-packed planes. This structure remains even after annealing at 573 K for 15 h. Only small magnetic contributions to the diffraction patterns are observed.