The physiological responses of two freshwater crustaceans, Asellus aquaticus L. and Gammarus fossarum Koch., following in vitro exposure to two pesticides were measured. Both species responded to short-term exposure with elevated levels of Respiration and/or lower levels of Electron Transport System (ETS) activity. 1 h exposure to concentrations of up to 10 mg L(-1) showed an effect in both test species. Laboratory tests confirmed that G. fossarum is more sensitive to short-term pesticide exposure than A. aquaticus. ETS/R ratio may be used as a quick predictor of effects on organisms exposed to pesticides.