Toxicological studies of four insecticides (malathion, carbaryl, bioresmethrin, and GH 74) against Musca domestica vicinia (Ampang strain) were undertaken with particular reference to age, sex and posttreatment temperature. It was found that bioresmethrin and GH 74, both with a negative temperature coefficient, have great potential for use against houseflies. In vitro inhibitory studies of head and body esterases showed that unlike malathion and carbaryl, bioresmethrin had only negligible effect on these enzymes. The possibilities of using bioresmethrin and GH 74 for controlling the housefly problem in the Cameron Highlands, West Malaysia are discussed.