Postlarval and juvenile prawns (Macrobrachium rosenbergii) were exposed to 7 mirex concentrations plus control for 96 hours. Static acute toxicity tests were conducted at 28 C using deionized water reconstituted to hardnesses of 11, 42, 160 and 300 mg/l as CaC03 with postlarvae and 42 mg/l with juveniles. No statistical differences were observed in survivals of postlarvae or calculated median lethal mirex concentrations among hardnesses. A 96-hour LC50 for all postlarvae was 33.9 µg/l and for juveniles 900.8 µg/l mirex. Postlarvae and juveniles exposed to 1000 µg/l accumulated mirex up to 513X and 88X control levels. Mirex residues in juveniles were greater in nerve tissue than abdominal muscle or remaining carcass.