A comparison is made between electron densities derived from the frequency of the plasma resonance and densities derived from the frequency of the beat between the electron gyrofrequency (ƒH) and the upper hybrid frequency (ƒT), over the density range 150 to 300 cm−3. Determinations were made from 77 ISIS 2 high-latitude ionograms on which both methods could be used. Densities derived from the ƒT−ƒH beat frequency were found to be lower than those from the plasma-frequency resonance by an average of 10%, with a dispersion of 10%. Although this result does not apply directly to electron densities of < 100 cm−3 for which the beat method is normally used, it indicates that the method is more accurate than might be supposed from theory applied to higher densities. The result also puts experimental constraints on the frequencies at which the ƒH and ƒT resonances occur.