Thomson scattering techniques were used to measure the frequency of plasma waves near the quarter‐critical surface in a well‐diagnosed plasma irradiated by a nanosecond CO2laser with intensity ∼1014W/cm2. The frequency &ohgr;pwas shown to be less than &ohgr;0/2, in disagreement with the commonly used estimate &ohgr;p=&ohgr;0/2+ (9)/(8) (ve/c)2&ohgr;0. The theory of Afeyan and Williams [Phys. Fluids28, 3397 (1985)] gives better agreement, and shows that the density scale length is more important than the temperature in determining the frequency shift.