Ionization currents in purified deuterium between plane parallel gold electrodes at separations up to 1·2 cm have been measured in the range 47 <or= E/p₀ <or= 165 v cm^-1 torr^-1 and 1·55 <or= p₀ <or= 18·4 torr where E is the applied electric field and p₀ the gas pressure (corrected to 273°K). The values of the primary ionization coefficient α/p₀ deduced by three methods from these measurements are compared with results of other workers. Within the limits of experimental error, α/p₀ was found to be a function of E/p₀ only.