AbstractThe interaction of Electron Cyclotron Resonant Heating (ECRH) with the m/n = 2/1 magnetohydrodynamic (MHD) mode, which was observed in toroidal plasmas, is attributed to the superthermal electrons which are produced on the EC resonance by the ECRH. Superthermal electrons diffusing across the q = 2 surface, exchange power with the m/n = 2/1 MHD mode which is either suppressed or enhanced. When the EC resonance is outside the rational surface the mode is always suppressed. When the EC resonance is inside the rational surface, modes with a large amplitude are enhanced while modes with a small amplitude are suppressed. It is also shown quantitatively that a superthermal electron population, three orders of magnitude below the thermal electron density, would be sufficient to suppress the mode and an experiment is proposed for the confirmation of the theory.