The freshwater prawn Macrobrachium dayanum was subjected to acute (96 hr) static bioassay toxicity test with the pesticide endosulfan and the LC50 values were 0.006 ppm. Prawns were exposed for 30 days under static condition to their sub-lethal concentrations viz. 0.003 ppm, 0.002 ppm and 0.001 ppm of Endosulfan. The growth was found to decrease in direct relation to increase in the concentrations of the toxicants. However there was an increase in growth at lower concentration of endosulfan.