It is demonstrated that radio‐frequency (rf) waves applied below the ion cyclotron frequency (&ohgr;/&OHgr;i≊0.75) can stabilize a mirror plasma against the interchange instability. A set of phased antennas in the Phaedrus‐B tandem mirror central cell [Phys. Rev. Lett.59, 206 (1987)] is used to select the rf azimuthal mode number (m). When them=−1 mode is selected, the ponderomotive force from the left‐hand polarized wave fields is sufficient to stabilize the plasma. Measurements of the rf mode number and of the wave polarization indicate strong excitation ofm=−1 modes with much of the wave having a left‐hand polarization.