Aspects of osmoregulation including salinity tolerance, osmoregulatory capacity, location of transporting epithelia, and the expression of the enzyme Na ϩ /K ϩ -ATPase were investigated in the developing brown shrimp, Crangon crangon (L.), from the North Sea. Early developmental stages and large juveniles were exposed to a wide range of salinities for measurement of hemolymph osmolality and survival rates. In media ranging from 17.0‰ to 32.2‰, salinity tolerance was generally high (survival rates: 70%-100%) in all developmental stages, but it decreased in media !10.2‰. Zoeal stages and decapodids slightly hyperregulated at 17.0‰ and osmoconformed in media ≥25.5‰. At 10.2‰, these stages showed high mortality, and only juveniles survived at 5.3‰. Juveniles hyperregulated at 10.2‰ and 17.0‰, osmoconformed at 25.5‰, and hyporegulated in media ≥32.2‰. Large juveniles hyperregulated also at 5.3‰. Expression of the Na ϩ /K ϩ -ATPase and ion-transporting cells was located through immunofluorescence microscopy and transmission electron microscopy. In zoeae I and VI, a strong immunoreactivity was observed in cells of the inner epithelia of the branchiostegites and in epithelial cells lining