The voltage (V) dependence of the mobility‐limited electron current density (J) in a gas‐filled diode is calculated for low pressure [(m/M)1/2&ngr;c/&ngr;p]<1 <&ngr;c/&ngr;pas well as for high pressure [1<(m/M)1/2&ngr;c/&ngr;p], wherem/Mis the ratio of electron mass to atom mass, &ngr;cis the average frequency of elastic collisions between electrons and gas atoms, and &ngr;pis the electron‐plasma frequency. It is assumed that &ngr;c(C) is proportional to the electron speedc, which corresponds to the case of an energy‐independent mean free path. It is shown thatJshould be proportional toV3/2p−1d−3for low pressure, and toV3/2p−1/2d−6/2for high pressure, wherepis the gas pressure anddis the (planar) diode spacing. Corresponding expressions for cylindrical geometry are derived and compared with experiment.