We report here measurements of dispersion and damping of plane electrostatic waves having phase velocities between 1.5 and 3 times electron thermal speed 3×107cm/sec. These waves were excited and detected at frequencies &ohgr;p<&ohgr;<2.5&ohgr;pusing gridded rf probes within an isotropic, quiescent, and collision‐free sodium plasma at thermal equilibrium. As plasma electron density (1×107to 2×108cm−3) and rf (30 to 180 MHz) are varied, the measured wavelengths and spatial damping rates were scaled to Debye length using independent Langmuir probe data. Full agreement is then found between the experimental results and the fundamental solution of Landau's dispersion relation.