We have generated, essentially instantaneously, a time‐reversed replica (phase‐conjugate) of a monochromatic optical wave (at 6943 A˚) by directing the wave into a CS2‐filled waveguide where it interacts via the third‐order nonlinear susceptibility with counterpropagating pump waves of the same frequency. This method is shown to have advantages over a similar replication process which has been observed with unguided waves in an infinite homogeneous medium.