The ontogeny of osmoregulation, the development of branchial transporting epithelia and the expression of the enzyme Na+/K+-ATPase were studied in Carcinus maenas (L.) from the North Sea, Germany. Laboratory reared zoea larvae, megalopae and young crabs were exposed to a wide range of salinities, and hemolymph osmolality was measured after 24 h exposure time (72 h in juveniles). Zoea I larvae slightly hyper-regulated in dilute media (10.2 and 17.0 ) and osmoconformed at >17 . All later zoeal stages (II IV) osmoconformed in salinities from 10.2 to 44.3 . The megalopa hyper-regulated at salinities from 10.2 to 25.5 . Young crabs hyper-regulated at salinities from 5.3 to 25.5 , showing an increase in the osmoregulatory capacity. The development of transporting epithelia and the expression of Na+/K+-ATPase were investigated by means of transmission electron microscopy and immunofluorescence microscopy. In the zoea IV, only a very light fluorescence staining was observed in gill buds. Epithelial cells were rather undifferentiated without showing features of ionocytes. Gills were found to be present in the megalopa, where Na+/K+-ATPase was located in basal filaments of the posterior gills. In crab I juveniles and adults, Na+/K+-ATPase was noted in the three most posterior pairs of gills, whereas anterior gills lacked the enzyme. Ionocytes could first be recognized in filaments of megalopal posterior gills, persisting through subsequent stages at the same location. Thus, the development of the gills and the expression of Na+/K+-ATPase are closely correlated with the ontogeny of osmoregulatory abilities. The morphological two-step metamorphosis of C. maenas can also be regarded as an osmo-physiological metamorphosis, (1) from the osmoconforming zoeal stages to the weakly regulating megalopa, and (2) to the effectively hyper-regulating juvenile and adult crabs.