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Relationships are obtained which allow to describe the characteristics of the twin as functions of time and medium constants for dynamic external loads within the framework of the dislocation theory. The case of large velocities is considered when the drag force arising in the dislocation motion is of the viscous friction character and, yet, the relativistic effects may be neglected. Two problems in the dynamic behaviour of the twin are solved, namely the escape of the elastic twin from a n unloaded crystal due to the surface tension force and passage across the crystal by the twin in a uniform stress field. The obtained results are compared with experimental data for iron, calcite, and cadmium. It is shown t h a t in the considered case of large velocities the twin motion velocity should be weakly (at least, not exponentially) dependent on temperature. This result is in agreement with available experimental data.

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