Parametric decay instability of an extraordinary electromagnetic wave into a magnetized electron plasma wave and a whistler wave propagating at an angle with respect to the static magnetic field has been considered. Application of the present investigation to tandem mirrors and tokamaks has been pointed out. For example (i) for the tandem‐mirror parameters at the second harmonic cyclotron resonance in the thermal barrier minimum, the convective threshold ∼100 W/cm2, (ii) for tokamak parameters when the extraordinary mode is launched from outside the torus, the convective threshold ∼1 kW/cm2for this decay process.