[en] In the positive column of a d.c. discharge a parametric instability has been excited by an antenna located outside the plasma. The frequencies and wavelengths measured are in accordance with the Manley-Rowe-conditions and identify the instability as a decay of a whistler mode into a high frequency lower hybrid mode and a low frequency (backward) ion cyclotron mode. The theoretical values of the growth rate agree well with those found in the experiment. An increase of ion temperature of about 40% is observed in the presence of the decay modes and is attributable to the parametrically excited ion cyclotron wave. (author)