Although magnetic field measurements have not been made on the surface of Venus or, for that matter, below the Pioneer Venus Orbiter (PVO) periapsis altitude of ~150 km, the theory of the large-scale ionospheric magnetization that results from the solar wind interaction can be used in concert with the available data to infer the existence of nonzero fields down to the surface. Here it is argued that magnetometer measurements in the upper part of the ''diffusion region'' of the ionosphere can be extrapolated to lower altitudes because of the particularly simple physics that determines the altitude profile of the large-scale magnetic field in this region. The magnetic field at the bottom of the ionosphere inferred by this method using the PVO data is up to tens of nanoteslas when large-scale ionospheric fields are present, suggesting the likely presence of fields of similar strength below. Given the negligible intrinsic field of Venus inferred from the PVO measurements, these fields must result from the solar wind interaction. The implications for Mars, which may have an analogous solar wind interaction, is that magnetometers on the surface could regularly observe varying fields of similar magnitude and origin. ¿American Geophysical Union 1991