The bioaccumulation potential of endosulfan was determined by constantly exposing male and female adult crayfish, Procambarus clarkii to 100 ppb Thiodan insecticide for 8 wks. The crayfish were removed at 2 wk intervals and the uptake by tissues (whole-body) was determined by electron capture gas-chromatography. The same number of pre-exposed crayfish (100 ppb Thiodan) were transferred to endosulfan-free water after 8 wks, and insecticide loss (depuration) was similarly quantitated. The maximum amount of insecticide and its metabolites detected after 8 wks were 197 ppb endosulfan II, 18 ppb endosulfan I and 3 ppb endosulfan sulfate. During the first 4 wks of depuration, endosulfan loss from cryfish tissues occurred rapidly, and only 3 ppb endosulfan I remained. No endosulfan II and sulfate were detected beyond 4 wks. The residues in male vs female were not significant statistically. Bioaccumulation factor (BF) for endosulfan II for crayfish tissues was 1.97, which is considerably lower than for other aquatic invertebrates (26 for scallop, Chlamys opercularis and 600 for mussel, Mytilus edulis. The presence of endosulfan sulfate in the tissues cannot be considered as 'detoxification' which is as toxic as the parent compound.