ZnS-ZnTe strained-layer superlattices (SLSs) were grown on (111) oriented BaF2 substrates by atmospheric pressure metal-organic chemical vapour deposition. Optical bistability (OB) with a counter-clockwise direction of the hysteresis loop in the ZnS-ZnTe SLSs has been observed at 77 K with nanosecond switching time for the first time. The origin of the OB was investigated by photoluminescence and absorption spectra measurement. The OB is attributed to band gap resonance non-linearity with density-dependent carrier lifetimes due to the band-filling effect in high-intensity photoexcitation.