Studies of the mobility of basal dislocations in zinc (1, 2) by use of a torsion stress pulsing technique (3) have shown that the maximum velocity of dislocations, V_(max), is a linear function of applied shear stress, τ, at stresses above 10^6 dyne/cm^2 as expressed by B V_(max) = τ b (1) where B is a drag coefficient and b is the basal Burgers vector. It has been concluded that some type of dislocation-phonon interaction is responsible for the damping of dislocation motion in an otherwise perfect crystal. Flow stress measurements have indicated a strong interaction between basal dislocations and forest dislocations (4). Hence, the scatter observed in the data of basal dislocation mobility measurements (1, 2) can be considered to be the result of interaction with non basal forest dislocations, whose density ranged between 10^2 and 10^4 cm^(-2). Recently, Frost and Ashby (5) analyzed the viscously damped motion of a dislocation through a regular array of discrete obstacles. Their calculations predicted that, at applied stresses higher than twice the critical stress to break through the obstacles (τ > 2τ_c), the motion of the dislocation is essentially governed by the viscous drag and not the obstacles. In the range τ_c < τ < 2τ_c, the average dislocation velocity is influenced by both the viscous drag and the obstacles. Existence of a critical stress for dislocation motion through random arrays of obstacles has also been predicted by a statistical analysis (6), and by a computer analysis (7).