The major objective of this work was to assess the possible long-term effects of Thiodan insecticide on the growth-rate, fecundity and hatching potential of Louisiana crayfish, Procambarus clarkii. Adult males and females were collected from an Experiment Station, and were exposed to 600 ppb Thiodan. Since the aqueous solubility of endosulfan (active ingredient of Thiodan) is very low, the actual concentration of 2, 10, 15 and 600 ppb Thiodan in tap water was determined by electron-capture gas chromatography. The amount of endosulfan I and II in 2 ppb Thiodan was 0.8 and 0.32 ppb, respectively; 10 ppb = 3.8 and 1.52; 15 ppb = 6.3 and 2.52; and 600 ppb = 284 and 113.6 endosulfan I and II, respectively. Male and female crayfish were exposed to 600 ppb Thiodan for 20 weeks during which mating occurred. The mean number of eggs produced by control crayfish (N = 10) was 288 and by treated crayfish (N = 10) was 225, which did not differ significantly. Hatchlings from unexposed crayfish were used to determine the effects of Thiodan on weight, length and molting frequency. Sixty hatchlings were exposed to 2, 10 and 15 ppb Thiodan for a period of 9, 23 and 27 weeks during which they achieved a mean weight (gain) of 0.64, 0.19 and 0.17 g, respectively. Control hatchling gained 0.82, 0.30 and 0.19 g during the same period. These differences were also insignificant statistically. The length (gain) by these hatchlings (control and treated) over the same time period did not differ significantly. There was a slight increase (insignificant) in the molting frequencies of treated hatchlings. Comparative data on the chronic effects of Thiodan for crayfish are not available. Based on the reported LC50 values for grass shrimp, mysid shrimp and Korean shrimp, we conclude that Louisiana crayfish, P. clarkii is more tolerant to this insecticide than the other decapod crustaceans.