The converse piezoelectric effect is used to suppress the vibrations of an othotropic plate stiffened with piezoelectric actuators. The control problem involves the minimization of the dynamic response of the plate by using the voltage applied to the piezoelectric stiffeners as a control variable. The dynamic response is defined as a combination of the dynamic deflection and the velocity of the plate and expressed as a quadratic cost functional The piezoelectric stiffeners are bonded on the opposite surfaces of the plate and placed symmetrically with respect to the middle plane. The control forces and moments are activated by applying in-phase or out-of-phase voltages, respectively. The active control takes the form of a piecewise constant alternating voltage with varying switch-over intervals to achieve optimum effect Numerical results are given to demonstrate the effectiveness of the proposed control mechanism and to investigate the effects of various problem parameters on the control of vibrations. Copyright © 1996 Elsevier Science Ltd.