Acute toxicity of an alkyl benzene sulfonate (ABS) surfactant detergent to five species of juvenile estuarine fishes was determined at 20‰ salinity and 20° C. Concentrations of detergent that will kill 50 per cent of the test organisms in 96 hours ranged from 7.0 parts per million (ppm) for silversides, Menidia menidia, to 22.5 ppm for mummichogs, Fundulus heteroclitus. Intermediate in susceptibility were mullet, Mugil cephalus (10.1 ppm), winter flounder, Pseudopleuronectes americanus (8.2 ppm), and eels, Anguilla rostrata (7.5 ppm). After 12 weeks in solution, the synthetic detergent (syndet) retained most of its toxicity. The stock syndet contained 30.3 per cent ABS at week zero as determined by the methylene blue method.