A host-parasite system, in which large and uniform populations of lice, Polyplax serrata (Burmeister), are developed on laboratory mice, was evaluated as a method for testing systemic insecticides. Tests with ronnel demonstrated that the effectiveness of multiple doses of a systemic insecticide could be evaluated by using this mouse-louse system. Furthermore, other studies on dosages and toxicity in mice would be possible. This host-parasite system has the advantage of testing the chemical on an ectoparasite that is living in its natural microenvironment.