Aspects of osmoregulation including salinity tolerance, osmoregulatory capacity, location of transporting epithelia, and the expression of the enzyme Na+/K+-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 hyper-regulated 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 hyper-regulated at 10.2 and 17.0 , osmoconformed at 25.5 , and hypo-regulated in media ?32.2 . Large juveniles hyper-regulated also at 5.3 . Expression of the Na+/K+-ATPase and ion-transporting cells were 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 the pleurae. Their ultrastructure showed typical features of ion-transporting cells. In decapodids and juveniles, ionocytes and expression of Na+/K+-ATPase remained located in the branchiostegite epithelium, but they disappeared from the pleurae and appeared in the epipodites. In large juveniles, the cells of the gill shaft showed positive immunolabeling and ultrastructural features of ionocytes. In summary, the adult pattern of osmoregulation in C. crangon is accomplished after metamorphosis from a moderately hyper-osmoconforming decapodid to an effectively hyper-/hypo-regulating juvenile stage. Salinity tolerance and osmoregulatory capacity are closely correlated with the development of ion-transporting cells and the expression of Na+/K+-ATPase.