The methods of comparative planetology are widely used for understanding the phenomena in Earth's ionosphere. In this paper we examine the reaction of the ionospheres of the Earth and Venus to an individual solar event − an X-ray / proton flare on 30 and 31 January 1982. The data from the Pioneer Venus and Dynamics Explorer-B satellites were used. This solar proton event was chosen because the position of the Earth and Venus at that time was such that it affected both planets. It is shown that both ionospheres reacted to the flare, but the reaction was much stronger in the nightside terrestrial ionosphere, and it was mainly seen in the convection zone, where the field lines of the Earth's magnetic field interact with the magnetic field of the solar wind. On Venus, which has no internal magnetic field, the phenomena observed in the nightside ionosphere are determined by the plasma transport from the dayside across the terminator and by the ion flows which leave the ionosphere, being accelerated in local induced magnetic fields. The initial conditions on Venus recover more slowly than on the Earth.