Results are summarized of a diagnostic study of underdense plasma targets, leading to the first observation of stimulated Brillouin scattering arising from thermal‐level fluctuations. Included are; (1) a detailed analysis of spectroscopic measurements of the plasma before and during laser heating, (2) computer calculations of plasma evolution during the laser pulse, (3) observations and anlaysis of Brillouin scatter, and (4) spectroscopic observations of forbidden satellite line emission during Brillouin scatter.