ABSTRACT.A screening programme was undertaken to examine the possibility of cross resistance occurring between cyromazine and conventional insecticides. The responses of nine strains ofMusca domesticato treatment with cyromazine, trichlorphon, methomyl and pyrethrins with piperonyl butoxide were measured. No tolerance to cyromazine was found, neither was there a direct correlation between larval responses to cyromazine and adult responses to other insecticides. Cyromazine is a potent larvicide againstM. domesticaand the results of these tests show that it has good potential for the control of houseflies with high levels of resistance to other insecticides.