Second‐harmonic generation (&lgr; = 5.3 &mgr;) is observed when dc fields and CO2laser beams are applied simultaneously to narrow band‐gap semiconductors. The nonlinear mechanism responsible for the dc‐induced second harmonic is shown to be a mobile‐carrier effect, and is ascribed to the nonparabolicity of the conduction bands. The possible application of this effect to the modulation of optical beams is indicated.