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. Salinity of the medium affects toxicity of the syndet. Eels which were exposed to 10 ppm syndet for 96 hours and mummichogs exposed to 20 ppm syndet for 120 hours were relatively unaffected at salinities below 15‰ but died at salinities approaching ocean strength. Prolonged exposure of 150 days of juvenile mummichogs to syndet concentrations of 1, 2, 5, 8, or 10 ppm at salinities of 10, 20, 30, or 40‰, when compared to controls, did not significantly affect any biological variable measured (mortality, growth in length and weight, red blood cell number, gonadosomatic index, liver condition).