Atomic short-range order is commonly observed in _CuMn alloys via a diffuse (1 �) peak in neutron diffraction patterns. There has been speculation for many years on the existence and nature of any corresponding long-range order. A sample of Cu80Mn20 has been irradiated in a high fast-neutron flux to produce a large number of crystal defects at around room temperature and thus facilitate the ordering process. Measurements of the diffuse neutron scattering from this sample demonstrate that long-range order has been induced in the sample, as evidenced by at least five Bragg peaks. This order is not seen in a reference sample prepared in identical fashion except for the irradiation. The Cowley short-range order parameters for both samples are calculated and a candidate structure for the long-range order is proposed.