The attenuation of 7 Mc/sec ultrasonic pulses in zinc single crystals before, during, and following plastic deformation has been measured using the ultrasonic pulse technique. Simultaneous measurements of the time-dependent plastic strain at constant stress were made in order to study the correlation between this strain and the attenuation. The crystals were oriented in such a way that the applied stress was a direct shear on the slip plane, thus causing the deformation to proceed only by slip over this plane. At the same time the high-frequency sound pulses were sent through the crystal perpendicular to the slip plane. It was found that the attenuation of transverse waves, whose stress vectors lay in the slip plane, was very sensitive to the deformation. Longitudinal waves, with stress vectors perpendicular to the slip plane, were substantially unaffected by the deformation. This indicates that the attenuation was caused by dislocations introduced with the deformation and moving only in the slip plane. The time-dependent changes in the attenuation and the strain enabled the motion of these dislocations to be studied. For small plastic strains, the attenuation of the transverse waves rose during the loading process, but decreased again after the load was completed even as the crystal continued to deform with time. For larger strains, the attenuation continued to rise after the load was completed and was nearly proportional to the strain. Upon unloading, the attenuation recovered very rapidly toward its value before deformation. If the unloading was carried out in steps, the attenuation decreased while the crystal rested under some intermediate load. When this load was decreased still further, the attenuation rose very rapidly until the unloading was stopped, after which time the crystal again rested under constant stress and the attenuation again decreased.