The one-dimensional propagation of stable negative-resistance domains is considered for the case in which the carriers interact with a single type of impurity level. Approximate expressions for the domain velocity and the current in the presence of a domain are obtained for the following cases:(i) Fast trapping - traps in equilibrium with the conduction band at all points in the domain.(ii) Slow trapping:(a) Zero trapping - domain moves too quickly for traps to respond.(b) Delayed trapping (I) - the high-field plateau is wide enough for traps to respond.(c) Delayed trapping (II) - traps in equilibrium with conduction band only above a critical field E_c.(c) is the case which most closely corresponds to the observed impurity-barrier negative resistance in Ge. The analysis is also relevant to the explanation of slow domains in n-type GaAs.