Observation of optical bistability at the band gap in InAs is reported. Clear hysteresis was seen in the reflected signal from a Fabry–Perot etalon consisting of polishedn‐type InAs with silver deposited on the back surface. By using the 3.096‐&mgr;m line of an HF laser, which matches the band gap at 77 K, bistable switching was achieved with power levels as low as 3 mW (peak intensity 75 W/cm2).