During 1971-1974 the susceptibility of the IVth larval instar of Aedes cantans, Ae. vexans, Ae. punctor, Ae. excrucians, Ae. communis, Ae. sticticus, Culex pipiens pipiens, Cx. pipiens molestus and Culiseta annulata to some organophosphates, carbamates and chlorinated hydrocarbons was estimated by means of the WHO method. The susceptibility to organophosphates such as chlorpyriphos, temephos, fenthion, fenitrothion, pirimiphos-methyl and bromophos was very high (LC 50 0.001-0.01 ppm), whereas dichlorvos, malathion, diazinon and trichlorphon were less toxic (LC50 0.03-0.24ppm). Carbamates such as carbaryl, propoxur and bendiocarb were of even lower toxicity (LC 50 0.09-0.38 ppm). No resistance to chlorinated hydrocarbons used in mosquito control in Czechoslovakia, such as DDT and lindane, was detected in the larvae studied. Of the chlorinated hydrocarbons methoxychlor was found in general to be less toxic than the rest.