A microwave method for determining the electron density in a high‐density plasma with or without a static magnetic field is presented. The method utilizes the cutoff frequency characteristic of a dielectric‐rod waveguide embedded in the plasma, the mode of propagation along the waveguide being such that it corresponds to the TMommode at cutoff. In certain cases the density can be calculated from simple formulae if the cutoff frequency is measured. The higher limit of the density that can be measured by usual microwave techniques can be increased considerably by using this method.