We report the observation of optical stability for a plane parallel semiconductor crystal which forms a Fabry-Perot interferometer using only the natural reflectivity of its surfaces. Nonlinear transmission is observed for cw laser intensities above ~ 100 W/cm2 for radiation at 1895 cm -1 near the energy gap of InSb at 5 K. The effect is interpreted in terms of a very large intensity-dependent refractive index giving a 5?/2 optical thickness change for an intensity of ~2 kW/cm2. Clear bistability is seen in fifth-order interference, the first such observation above first order in an intrinsic, one-element system, in addition to regions exhibiting signal amplification. The same crystal also shows strong modulation of the transmission of one laser beam induced by a second, with real signal gain, thus demonstrating an "optical transistor.".