Osmoregulation was studied in the zoeal stages I and VI, the first decapodid, the first juvenile, and in adults of the palaemonid shrimp Palaemonetes argentinus. The larvae hatch in freshwater creeks or in adjacent brackish coastal lagoons of the warm temperate southwestern coast of the Atlantic Ocean; larval development is possible in low salinities. To cope with these demanding environments, the capacity for osmoregulation is well developed at hatching, increasing only slightly throughout development. All the postembryonic developmental stages hyper-regulated at low salinity (1 to 10 parts per thousand), hyper-osmoconformed at 17 parts per thousand, and osmoconformed at higher salinities (26 parts per thousand; up to 32 parts per thousand in adults). The type of osmoregulation did not change during development from larval hatching through the adult phase. The ecological implications and the evolutionary significance of osmoregulation in early life- history stages of P, argentinus and other aquatic crustaceans are discussed.