Acute (96-h) flow-through toxicity tests with endosulfan (Thiodan) were conducted with several estuarine animals. The test species and their 96-h lethal concentration for 50 percent of the organisms (LC₅₀) values were: pink shrimp (Penaeus duorarum), 0.04 μg/litre; grass shrimp (Palaemonetes pugio), 1.3 μg/litre; pinfish (Lagodon rhomboides), 0.3 μg/litre; spot (Leiostomus xanthurus), 0.09 μg/litre; and striped mullet (Mugil cephalus), 0.38 μg/litre. In a 56-day bioconcentration study (28-day uptake, 28-day depuration), striped mullet were exposed to 0.008 and 0.08 μg endosulfan/litre seawater. The two endosulfan isomers (endosulfan I and II) were rapidly metabolized to endosulfan sulfate; only trace amounts of each isomer were detected in edible tissue or offal of mullet exposed to 0.08 μg/litre (0.035 μg/litre measured) for 28 days. Maximum bioconcentration factors of endosulfan were 2249 in edible tissue and 2755 in whole-body analyses. After 48 h in pesticide-free seawater, endosulfan was not detected in the previously exposed mullet. Our studies suggest that endosulfan in the estuarine environment would be a hazard because of its acute toxicity and bioconcentration potential, but animals surviving exposure and moving to areas free of endosulfan would lose the chemical rapidly.