Optical bistability in the ZnSe/ZnS multiple quantum wells (MQWs) grown by MOCVD has been observed at 77 K for the first time with ns switching time. On the bases of the shape of hysteresis loop and photoluminescence and absorption spectrum measured, the optical bistability in the ZnSe/ZnS MQWs can be attributed to enhanced absorption effect, which originated from the collision broadening of excitonic resonance.