Laboratory and field tests were conducted to determine the relative effectiveness of malathion-resmethrin mixtures and malathion only. Laboratory tests conducted with malathion susceptible species showed a slight increase in effectiveness of a 90:1 malathion-resmethrin formulation over malathion for Culex nigripalpus, but not with Aedes taeniorhynchus. With resistant Ae. taeniorhynchus, there was no significant difference between the 100:1 malathion-resmethrin formulation and malathion. In the field tests, both technical and dilute formulations were used in test against Cx. nigripalpus, Cx. quinquefasciatus and Ae. taeniorhynchus. These tests showed no difference in mortality between any of the malathion-resmethrin formulations used and malathion, but indicated that the differences in mortality obtained were dependent on the actual amount of malathion discharged regardless of the addition of resmethrin.