A method is described for the measurement of plasma ion density which relies upon the attenuation of a medium energy atomic beam. For beam energies of a few keV, resonant charge transfer with plasma ions predominates so strongly over competing processes leading to beam attenuation that the determination of ion density is particularly simple. We have employed a 2.5—5-keV atomic hydrogen beam to measure the proton densities produced by a pulsed plasma gun and by the B-l stellarator. In the latter case a 4-mm microwave interferometer was used to confirm the calculated calibration. The time response of the system is of the order of 1 MHz for density changes ≥ 1013/cm3. The method appears particularly well suited for ion densities of order 1014/cm3 where microwave measurements are difficult.