In a magnetoplasma, the amplitude of the lower oblique resonance in the mutual impedance of two electric antennas is very sensitive to the plasma frequency. This property can be used to measure the electron density in a simple way when the electron temperature and the magnetic field are known, provided also that the probe has been properly calibrated. As an application, densities are deduced from measurements made by a mutual‐impedance probe on a rocket in the auroral ionosphere. In the future, better practical design and theoretical modeling of the probe should make calibration unnecessary.