The influence of selected environmental conditions on the acute toxicity of 3 selected chemicals (K2Cr2O7, CuSO4 and Na pentachlorophenate) to instar II-III larvae of Streptocephalus proboscideus was evaluated. Mortality after a 24-h incubation was used as effect criterion. Factorial experiments were set up resulting to 25 different combinations of temperature and water hardness. The effects of pH of the dilution medium and light regime during toxicant exposure were also assessed. Results (24-h LC50's) from nearly 200 tests revealed that the degree of variation is chemical- and test condition-dependent. Water hardness had the greatest effect on larval sensitivity, showing a significant (p<0.05) negative correlation. A significant effect (p<0.05) on sensitivity was furthermore detected only for the extreme values of temperature (30 degrees C) and pH (6.0) tested. Light conditions during acute exposure caused no change in larval sensitivity.