The existence of dineutron was predicted 75 years ago. At present a number of experimental works confirm this prediction. Owning to the principle of isotopic invariance a singlet deuteron must also exist. The possibility of describing the neutron-proton interaction in a state at low energies as the excitation of a quasi-stationary level below the deuteron decay threshold (singlet deuteron) is discussed. The position, neutron and radiative widths of the level are determined by the scattering length, effective radius and cross section for radiative capture of neutrons by protons. Possible experiments to find this level are discussed. Discovery of a singlet deuteron would be confirmation of the existence of dineutron.