Single crystals of nominally pure KCl and KCl doped with Mg2+,Ca2+,Sr2+ or Ba2+ were deformed by compression at 77-254 K; during the tests strain-rate cycling was conducted in association with ultrasonic oscillation. The data were analyzed in terms of strain-rate sensitivity ((Deltatau'/Deltaln (epsilon) over dot)) versus stress decrement (Deltatau). The curve for KCl doped with the divalent impurities has two bending points and two plateau regions. It is proposed that the variation of strain-rate sensitivity at the second plateau place on the curve with shear strain (Delta(Deltatau'/Deltaln (epsilon) over dot)) is due to a change in forest dislocation density with shear strain. The forest dislocation density for the specimens seemed to increase by the divalent additions in the compression test on account of the jogs on the screw dislocations. It depended on the concentration of impurities and also on the size of impurity in the specimens at a given temperature. Unfortunately, it was not possible to determine whether a change in the size of impurity influences mobile dislocation density, rho, from the values of Deltarho/Deltatau' for KCl doped with Ca2+,Sr2+ or Ba2+. (C) 2003 Kluwer Academic Publishers.