Toxicities of DDT, chlorpyrifos, and fenthion to Cyprinotus incongruens Ramdohr and Chlamydotheca arcuata Sars were evaluated in laboratory cultures. For the 1st species, the series from most to least toxic was chlorpyrifos, fenthion, DDT; and for the 2nd species, it was chlorpyrifos, DDT, and fenthion. Data for 24 h were inadequate to predict field effects because mortalities in test plates increased each day, even at concentrations where no 24-h mortalities were recorded. Temperature stress which would be likely to occur in early spring and in late summer (5 and 35°C) increased insecticide toxicities at a given concentration; lowest mortalities occurred at intermediate temperatures (15 and 25°C). There is some tendency towards induction of egg-laying in these 2 species by toxic doses of the insecticides; the effect is even more pronounced in Cypridopsis vidua O. F. Muller. At all intermediate doses, some fraction of the eggs hatched and matured, which could be a mechanism for reestablishment of populations, but at the highest DDT doses (5.0 ppm) 0 eggs hatched.