A torsional hydromagnetic wave is induced by application of an oscillatory radial electric field to one end of a cylindrical deuterium plasma located in a magnetic mirror field. The wave propagates in the axial direction to the center of the device, where a resonance occurs between the hydromagnetic wave frequency and the local ion cyclotron frequency. Wave magnetic‐field measurements indicate that transfer of power from the 1‐MW, 8.3‐Mc oscillator to the hydromagnetic wave is about 65% efficient, and that more than 90% of the wave energy is transferred out of the wave at the resonance region.