A method for determining electron density in cylindrical plasma columns uses surface wave resonances which occur when the length of plasma column is an integral multiple of the half‐wavelength of the surface wave. One type of resonance is in an axially symmetric mode, the other in a dipolar mode. A theory is given for the relation of resonance frequency to electron plasma frequency. Experiments were carried out with both propagation modes excited simultaneously; the axially symmetric resonances are at much lower frequencies than the electron plasma.