The damping constantBfor the viscous drag on dislocations was measured for NaCl and LiF. A novel technique was employed to improve the accuracy of the measurements, and many specimens were used to check the reproducibility of the results. Measurements of both the attenuation and velocity changes resulting from dislocation pinning by cobalt‐60 gamma irradiation were made. The values obtained areB=2.4 and 1.6×10−4in cgs units for LiF and NaCl, respectively. These values compare favorably both with theoretical estimates and with values determined from direct measurements at higher stresses by etch‐pit techniques. From the ultrasonic measurements, it is deduced that the viscous drag force is linear in the velocity and that relativistic velocities cannot be achieved in LiF and NaCl at room temperature at the yield stress of soft crystals. The results found demonstrate that relatively precise values for the dislocation drag can be obtained ultrasonically, that values for the dislocation densities in specimens can be determined from nondestructive ultrasonic measurements, and that the ultrasonically determined dislocation velocities are directly related to those determined by etch‐pit measurements at higher stresses.