Several synthetic pyrethroids evaluated against 4th stage larvae in the laboratory were found to be highly effective. The LC90 range was 0.045-8.0 ppb. Among these, FMC-30980 [(+) (-Cyano-3-phenoxybenzyl) (+)-cis-trans 2,2 dimethyl-3-(2,2-dichlorovinyl_ cyclopropane- 1 - carboxylate], FMC - 45497 (+ cis-isomer of FMC-30980), and FMC-45498 or decamethrin ( + Cis and bromo analogue of above) showed the highest activity with the latter being the most active compound. In the field, FMC-33297 or Pounce [3-phenoxybenzyl ( + ) Cis-trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane-1 carboxylate] and SD-43775 or Pydrin [a-cyano-3-phenoxybenzyl-4- chloro- a - (1-methylethyl)-phenylacetate] yielded good to excellent control of multi-resistant strains of Aedes nigromaculis Ludlow at 0.01 and 0.025 Ib/acre (11 and 28 g/ha), respectively. FMC-45498 was the most effective compound, producing excellent control of Ae. nigromaculis and Psorophora confinnis (L-A) at 0.001-0.0025 Ib/acre (1.1 and 2.8 g/ha). Against a 3 species of stagnant-water mosquitoes, FMC-45498 again was the most effective compound. It produced 80% control of these species for 2 wk at 0.001 Ib/acre (1.1 g/ha). Control was more long-lasting in the cooler months than in the warmer months. All the pyrethroids proved toxic to mayfly populations in the field at larvicidal rates. Recovery of populations occurred 2-3 wk after application in some of the treatments. SD-43775, however, was the most toxic, suppressing mayfly and dragonfly naiads for more than 3 wk. Dragonfly naiads were not adversely affected by larvicidal rates of the other compounds.