Fenitrothion was very effective 24 hr. posttreatment against both Psorophora columbiae (Dyar & Knab) and Anopheles quadrimaculatus Say at 236.5 ml/min in open canopy, but was not effective against An. quadrimaculatus in heavy canopy. At 24 hr., a dosage of 59.1 ml/min, fenitrothion was very effective against Ps. columbiae in the open at all swath widths. This dosage did not penetrate the canopy to control An. quadrimaculatus. Knockdown at one hr. posttreatment was poor for both species in all instances, averaging less than 31%. Propoxur (.757 kg ai/3.79 liter) killed 100% of caged mosquitoes at all swath widths and canopy cover at dosages at 177.4 ml/min., 24 hr. posttreatment knockdown at 1 hr. posttreatment averaged above 95%. At rates of 177.4 ml/min. and higher, propoxur penetrated heavy canopy affording good control . Propoxur (.454 kg ai/3.79 liter) effectively controlled both mosquito species at all swaths and times. Sevin (carbaryl) 4 oil effectively controlled Ps. columbiae at all dosage rates only at distance 0. The rate of 236.5 ml/min. gave the only satisfactory results against A. quadrimaculatus at 0 distance. Sevin 4 oil did not seem to be well adapted for ultra low volume (ULV) applications.