The excitonic optical bistability with picosecond switching time in ZnSe-ZnTe/GaAs multiple quantum wells(MQWs)has been studied on reflection at room.temperature for the first time.The experimental results indicate that the switching threshold from high to low states and the contrast ratio for the optical bistability are about 1.1MW/cm

2

and 6:1

respectively.the major nonlinear mechanism for the optical bistability is due to the change of refractive index caused by the excitonic saturating absorption.