A concentration of 0.5 ppb of the light-stable pyrethroid, Roussel-Uclaf 22974; NRDC-161; or Decis (S-[cyano(3-phenoxyphenyl) methyl] cis - (+) - 3 (2,2 - dibromoethenyl -2 -2 - dimethylcyclopropanecarboxylate) gave complete kill of 4th-instar larvae of Culex quinquefasciatus Say in laboratory tests at ca. 24.4 C (76 F). The LC50 was 0.14 ppb. In semifield tests in 1 liter of water [plastic containers and daytime temperatures of ca. 37.8 C (100% F ) all 4th-instar Cx. quinquefasciatus larvae were killed by a concentration of 1 ppb; the LC50 was 0.45 ppb. In semifield tests in 1-m2 sod-lined ponds (daytime temperatures of ca. 37.8 C) all larvae of Cx. tarsalis Coquillett, Cx. peus Speiser, and Cx. quinquefasciatus were killed at a rate of 8.41 g/ha (0.0075 Ib/acre). A minimum residual effectiveness of 15 days in the ponds was obtained at rate of 11.21 g/ha (0.01 Ib/acre).