A general expression for the group refractive index of the ionosphere has been deduced. Considering the relevant quadrants for the phase-difference between the normal and the abnormal components of the magnetic vector of the radio wave, the general expression gives the values of the group refractive index for the ordinary and the extraordinary modes for different frequencies and for different electron-densities and electron-collisional frequencies of the ionospheric medium and also under different conditions of the earth's magnetic field. © 1961.