A complete experimental and theoretical account is given of the eigenmodes (cold and hot) which exist at low power in a cavity partially filled with a plasma. The cavity modes which are perturbed by the plasma together with the basic cold‐plasma modes are studied theoretically using uniform cold‐plasma theory. The hot‐plasma modes of the system are investigated on the basis of a WKB approximation of the relevant equations of a nonuniform hot plasma. Experiments performed with a spherical discharge tube and also with a spherical afterglow plasma give results which are in satisfactory agreement with theoretical predictions. The measuredQfactor of the resonances also agrees with theoretical calculations.