Summary: We consider antiplane shearing motions of an incompressible isotropic visco-plastic solid. The flow rule employed is a properly invariant generalization of Coulomb sliding friction and assumes a constant yield stress or threshold above which plastic flow occurs. In this model stresses above yield are possible; but when this condition obtains, the plastic flow rule forces the plastic strain to change so as to lower the stress levels in the material and dissipate energy. On the yield surface, the flow rule looks like the classical one for a rate independent elastic-perfectly plastic material when the velocity gradients are small enough but differs from the classical model for large gradients.