Eighteen organophosphorous (o-p) insecticides and DDT, as either emulsifiable concentrate or wettable powder formulations, were evaluated for larvicidal activity under laboratory conditions simulating natural breeding sites in Czechoslovakia. All 18 compounds were tested against Culex pipiens molestus, and 8 o-p's and DDT against Aedes cantans. Temephos and chlorpyrifos were the most toxic to C. p. molestus larvae (LC100 0.002 ppm). Bromophos, pirimiphos-methyl and coumaphos were also highly toxic (LC100 0.01 ppm). Among 8 o-p's tested under field conditions (flood plain forest), temephos (LC100 0.0005 ppm), chlorpyrifos and bromophos (LC100 0.01 ppm) and pirimiphos-methyl and fenitrothion (LC100 0.02 ppm) had the highest larvicidal action against young 4th-instar larvae of Ae. cantans. The initial larvicidal activity of o-p compounds was reduced by the presence of decaying organic material. However, mud and water plants exerted only a negligible influence upon initial kill by fenitrothion, pirimiphos-methyl and temephos. The LC100 range for temephos, pirimiphos-methyl and fenitrothion against all larval instars of Ae. cantans was 0.005-0.02, 0.02-0.05 and 0.02-0.1 ppm, respectively. Pupae showed low sensitivity (LC100 0.5 ppm) to these compounds. The residual effectiveness of fenitrothion, pirimiphos-methyl and temephos applied at doses giving close to 100% kill of Ae. cantans larvae was 3, 5 and 3-7 days, respectively. Under laboratory conditions simulating natural breeding sites, the residual effectiveness approached these values when water temperatures were 20C. Presence of water plants greatly reduced residual effectiveness. When fenitrothion, pirimiphos-methyl and temephos were used at doses giving close to 100% control of the young 4th-instar Ae. cantans larvae, most nontarget organisms survived (Rhynchelmis spp. [a water worm], Mollusca [water snails], Asellus aquaticus [an isopod], Hydrachna spp. [water mites], Gerris lacustris [water striders] and Dytiscidae [predaceous diving beetles]). Only with Chirocephalopsis grubii (a phyllopod) was mortality 100%. Trichoptera (caddis fly) larvae were very sensitive to fenitrothion and pirimiphos-methyl, as were Cyclops spp. (Copepods and Daphnia spp. (water fleas) to temephos and pirimiphos-methyl. The latter insecticide was also highly toxic to ostracoda.