Two instability mechanisms which could occur under the conditions found in a perpendicular collisionless shock wave are considered. Both mechanisms are due to the negative energy character of the electron Bernstein waves propagating in the direction of the current flow in the shock. The effect of the voltage jump through the shock, and the density and magnetic field gradients at the shock front are considered. Both instability mechanisms occur only within a definite band ofkvalues. The first mechanism is due to a resonance between the ion acoustic wave and one of the Bernstein harmonics and the second to resonant ions absorbing energy from a negative energy Bernstein mode. The first case requiresTe≫Tiwhereas the second case can occur for arbitrary values of the ratioTi/Te, although the maximum effect occurs when this ratio is of order unity.