Nine pyrethroids, both alone and synergized with piperonyl butoxide, were tested as wind-tunnel-generated aerosols against field-collected females of Anopheles quadrimaculatus Say and Psorophora columbiae (Dyar and Knab). All pyrethroids tested were more effective than malathion and their toxicity was enhanced by combination with piperonyl butoxide. The reciprocal LC-90 ratios to malathion for the unsynergized pyrethroids ranged from 11.9 to 313.6 against An. quadrimaculatus and from 1.3 to 25.4 against Ps. Columbiae. The least effective unsynergized pyrethroid, fenvalerate, was enhanced the most (5.2X=8.8X) by the synergist. The malathion standard had an LC-90 of 2195 ug/ml against An. quadrimaculatus and 355 ug/ml against Ps. columbiae.