In this paper we report the development of the single frequency dye laser as a plasma diagnostic for measuring ion velocity distributions with a resolution limited only by the natural linewidth of the laser excited resonance transition. For the 6S1/2–6P1/2transition in Ba+this corresponds to a velocity uncertainty of 103cm/s. Velocity selection is performed by the laser as it scans the Doppler broadened plasma absorption line, in contrast with the usual method of scanning an emission line with a high‐resolution spectrometer. Both sensitivity and resolution are improved by 2 orders of magnitude, allowing nonperturbing measurements to be made at densities below 107cm−3. In addition to a description of the technique, sample measurements of time‐resolved wave induced modifications of ion velocity distributions are shown.