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 ?(3) of the order of 10-4 esu over the range of intensities investigated.