Measurements of N2, Ar, O, and He densities obtained by the gas analyzer on board the satellite Esro 4 indicate in both hemispheres a persistent longitudinal variation of thermospheric composition in middle and high latitudes. Increases of N2 and Ar densities are strongly correlated with a reduction of He and O densities. The amplitudes of the variations are higher in the southern hemisphere than in the northern hemisphere. Universal time variations are only weakly observed in polar latitudes. We present an empirical model, which describes the latitudinal dependence of longitudinal composition variations under quiet geomagnetic conditions.