We report the observation of optical bistability in an InSb resonator at room temperature. This effect and fringe shifts were caused by nonlinear refraction induced by two‐photon absorption of radiation from a single longitudinal mode injection‐locked pulsed CO2 laser operating at 9.6–10.6 μm. Intensities as low as 100 kW/cm2 were found to be sufficient to tune the 250‐μm‐thick cavity through a fringe maximum. From our results we deduce a value of χ(3) of the order of 10−4 esu over the range of intensities investigated.