It is shown that noise from light‐scattering fluctuations inherent in an optical Kerr medium sets a lower limit on the incident probe power required to achieve meaningful phase conjugation by four‐wave mixing. For visible wavelengths and room temperatures, we determine that about 10 &mgr;W of probe power is needed to obtain a signal‐to‐noise ratio of unity in the conjugate wave for reflectivities that are less than or near the first oscillation condition.