A study of the mobility of both whole and twinning dislocations is essential to our understanding of the mechanisms of deformation by slip and by twinning. Individual twinning dislocations are difficult to reveal, however, and the velocity of twinning dislocations has been determined only in calcite [i]. Measurements of the rate of motion of t~vin bands in the tangential and normal directions have been made on many materials. In the static loading of zinc single crystals, rates of broadening of a twin band of only one order of magnitude were obtained, namely from tenths of a millimeter per second to a few millimeters per second [2, 3]. The rate of broadening of a twin band has also been determined on zinc single crystals under static loading In [4], but In this case the rates (0.2-5 9 10 -2 ram/see) were two orders of magnitude less than those mentioned above. Rates for the tangential growth of twins in iron crystals under dynamic loading were found in [5, 6, 7] to be i000, 1400, and 2500 m/sec , respectively.