This paper reports experiments on initially quasi-conical premixed flames subjected to acoustic forcing of the cold gases with high amplitude and high frequency (≈ 1000 Hz). The resulting hemispherical-shaped flame, induced by a restabilization effect described previously by Duroxel al., shows a parametric instability when stronger acoustic forcing is applied (the oscillating acceleration is typically about l000g). In this case, the front exhibits unsteady cusp-shaped cells. These patterns oscillate with a period which is twice the acoustic period. This state is identified as the parametric response of a harmonic oscillator.