AbstractFlow‐through and static tests were conducted with fathead minnows, rainbow trout andDaphnia magnato determine their relative sensitivities to measured concentrations of the insecticide endosulfan and to compare responses of fish in replicated static and flow‐through exposure procedures. Fathead minnow 96‐h static LC50values were 1.3, 0.8 and 1.3 μg/L endosulfan. The 96‐h LC50values for fatheads in flow‐through tests were 1.7 and 1.0 μg/L. Rainbow trout 96‐h static LC50values were 1.7 and 1.6 μg/L. The 96‐h LC50values for rainbows in flow‐through tests were 0.3 and 0.4 μg/L. Acute 48‐h static EC50values forD. magnawere 343 and 271 μg/L endosulfan. These results suggest that differences in species sensitivity can be as great as thr