Internal friction measurements made as a function of strain amplitude on deformed sodium chloride single crystals were followed by elastic modulus measurements made during x irradiation of the same crystals. Etch pit densities were also determined. These data were used to evaluate the constants in a theory, proposed by Granato and Lu¨cke in 1956, which permits the calculation of the average length of the dislocation segments,L, which vibrate under an applied alternating stress. An independent determination of the magnitude ofLwas made from the elastic modulus data using a method developed by Gordon and Nowick.It is concluded that the theories tested in this paper give a useful representation of dislocation damping phenomena in sodium chloride and permit use of internal friction measurements for the continuous observation of dislocations.