Abstract A monokinetic neutron beam has been developed. The beam is produced by bombarding a liquid deuterium target with protons. By varying the proton energy the neutron energy is changed; energies from 50 to 150 MeV can be obtained. The neutron beam has an energy width of between 6 and 10 MeV. The main contribution to the width is the energy loss of the protons in the deuterium target. At full energy the neutron intensity is 4 × 10 4/sec. The beam greatly simplifies neutron-proton scattering experiments and also enables one to undertake nuclear spectroscopy.