A 6-keV beam of potassium atoms has been used to measure the integrated line density of a deuterium plasma in which the electron temperature was in the range of a few hundred eV. and the mean ion energy was in the range of 5–10 keV. Line densities from 5 × 1011 to 1014 particles per cm2 can be effectively measured. The various contributions to the attenuation of the neutral beam have been examined in sufficient detail to permit an evaluation of this measurement to a wide range of plasma conditions.