This paper deals with cylindrical cavity resonators containing weakly ionized gases, principally argon, in the pressure range of 0.03–0.70 Torr. The plasma, produced either by microwave or dc power, is magnetized by a uniform axial field. Measurements of resonant frequency, made over a range of plasma densities and magnetic field, are compared with the calculations of the theory given by Kent [J. Appl. Phys. 39, 5919 (1968)]. The agreement is good providing the cylindrical degeneracy of the cavity is preserved and the plasma is reasonably uniform. Experimental and analytical results are used to evaluate perturbation theory, and some simple formulas, useful for diagonostic purposes, are given. A perturbation method for correcting the theory to account for symmetrical nonuniformities is proposed, and it is applied to the experimental data with apparent success. The method offers the possibility of determining both uniformity and density from measurements of frequency in one microwave band.