In the laboratory, 5 new pyrethroids were screened against larvae and pupae of Culex quinquefasciatus. Two of the materials proved highly active, yielding 90% mortality in larvae at 0.07-0.46 ppb. Three of the materials were highly active against pupae, yielding 90% mortality at 1-4 ppb. Two of the most active pyrethroids (cypermethrin and S-3206 or fenpropathrin: Cx cyano-3-phenoxybenzyl 2, 2, 3, 3-tetramethyl cyclopropanecarboxylate) were evaluated under field conditions against larvae and pupae of Cx. tarsalis and Psorophora columbiae. Cypermethrin produced 90-100% mortality in larvae of the former species at 3-5 g AI/ha while S-3206 yielded 90-100% control at 27-55 g AI/ha. Both materials were equally effective against the second species, yielding high level to complete control of larvae at the low rates of about 6-11 g/ha. These rates also produced high level to complete control of pupae. The activity of these materials against pupae provides a decided advantage for use of these materials in mosquito control programs. Under field conditions, both cypermethrin and S-3206 showed no adverse effects on diving beetle adults. Cypermethrin was also relatively innocuous to dragonfly naiads and ostracods at effective larvicidal rates. However, this material reduced mayfly naiads to a very low level and recovery was not noticeable 2 weeks later. S-3206 adversely affected both mayfly and dragonfly naiads, but had no effects on ostracods except for a short period. The affected organisms recovered in 2 weeks after treatment.