The paper contains the data on temperature and concentration dependences of the direct current conductivity (sigma(DC)) of the electrolyte composed of dimethyl sulfoxide (DMSO) + ammonium nitrate (NH4NO3). The measurements were carried out in the whole solubility range of the nitrate in DMSO: 0 < x(NH4NO3) < 0.4 (x is the mole fraction) and in the temperature range from (293.15 to 333.15) K. In the isothermal conditions, the conductivity dependence on concentration shows a maximum sigma(max) at the nitrate mole fraction x(max), and the linear dependence of sigma(max) vs x(max) was found in the studied concentration and temperature ranges. The conductivity dependence on the nitrate mole fraction in DMSO, sigma(DC)(x), was perfectly fitted with the Casteel-Amis equation.