Flow-through acute toxicity tests were conducted with 24 organic compounds using fathead minnows Pimephales promelas as test organisms. The tested toxicants consisted of 11 substituted phenols, four substituted benzenes and nine esters. The 96-h LC50 values determined for these compounds when tested against fathead minnows ranged from 0.135 mg litre-1 for nonylphenol to 117 mg litre-1 for nitrobenzene. Adverse sublethal effects were observed in fathead minnows at exposure concentrations below the 96-h LC50 value with 14 of the 24 compounds tested. Acute toxicity tests were also conducted with 2,3,4,5-tetrachlorophenol using rainbow trout Salmo gairdneri; with 1,4-dinitrobenzene, 2-ethoxyethylacetate, ethyl salicylate and phenyl salicylate using both channel catfish Ictalurus punctatus and snails Aplexa hypnorum; and with 4-nitrophenol using channel catfish.