The Brillouin backscattering instability is studied for a range of preformed plasma conditions and using a CO2 laser in the 1012–1013 W/cm2 regime. A short prepulse is incident on a planar target ahead of the main‐pulse to produce the preformed plasma. The instability appears in a short burst of back‐reflected light. Saturation of the backscatter level is observed for an individual increase of the prepulse energy, main‐pulse intensity, and prepulse‐to‐main‐pulse delay. Ion Landau damping is strong and average back‐reflected intensities are limited to less than 30% of incident. Backscattered light spectra suggest that the critical surface is involved in the backscatter process when the laser beam is at best focus onto the target surface. Otherwise, the spectral signature is similar to those obtained from underdense plasmas.