The threshold intensity for excitation of the stimulated Raman backscatter instability has been measured at 9.58 &mgr;m in a well‐diagnosed plasma free of Brillouin scattering. The initial level of density fluctuations was independently measured by ruby laser scattering. The observed threshold of 1×1011 W/cm2is somewhat below the calculated ‘‘absolute’’ threshold for underdense plasmas, and the apparent noise level is higher than that measured directly.