The United States Environmental Protection Agency routinely use the sub-tropical mysid Americamysis (=Mysidopsis) bahia (Peracarida: Mysidacea) in laboratory testing to measure the toxicity of contaminants and to assess the potential hazards of toxic chemicals to the natural aquatic environment. The use of A. bahia in laboratory toxicity testing has extended to Europe, however, there is increasing interest in using indigenous species for predicting toxicity hazards to European waters. Neomysis integer (Leach) is the most common member of the hyperbenthic fauna of Western European estuaries and has been proposed as an alternative to the standard American species (A. bahia) for toxicity testing. Adult N. integer were exposed to chlorpyrifos and dimethoate (both widely used organophosphate pesticides) in flow-through and semi-static acute toxicity tests. Neomysis integer was more sensitive to chlorpyrifos (96h LC 5 0 of 0.13μg chlorpyrifosl - 1 ) than dimethoate (96h LC 5 0 of 540μg dimethoatel - 1 ). Juvenile N. integer, collected within 24h of release from ovigerous females, were equally tolerant to chlorpyrifos as adult mysids (96h LC 5 0 of 0.19μg chlorpyrifosl - 1 ). Gas chromatographic analysis of chlorpyrifos in the water samples revealed that considerable degradation or loss (>50%) occurred within 24h in the semi-static exposure vessels, highlighting the advantages of flow-through testing facilities. Although the 96h LC 5 0 for chlorpyrifos was higher for N. integer than for A. bahia (e.g. 0.035μg chlorpyrifosl - 1 ), it was comparable with those of other frequently-used test species (0.1-6.0μg chlorpyrifosl - 1 ). The results support the use of N. integer as an alternative to A. bahia for the testing of toxicants and the evaluation of toxic contamination events in European estuaries.