Parametric amplification of acoustic noise has been studied for high‐amplitude surface and bulk waves in photoconducting CdS. In contrast to the situation at low amplitudes, the amplified noise is collinear with the pumping wave and is contained in an extremely narrow band around one‐half the pump frequency. These results, as well as the observed voltage and time dependence, can be accounted for by the tight carrier bunching that occurs at high wave amplitudes.