The ontogeny of osmoregulation was studied in laboratory-reared early developmental stages of the Chinese mitten crab, Eriocheir sinensis, from the Elbe estuary (North Sea, Germany). At salinities ranging from 0.1644.3 , survival rate was quantified, hemolymph osmolality was measured, and osmoregulatory capacity was calculated as difference between osmolality of the hemolymph and the external medium. Zoea larvae hyper-regulated in dilute media, but osmoconformed in seawater and at higher salinities (≥32.2 ). Megalopae and stage I-II juveniles hyper-regulated at low salinities and hypo-regulated at ≥32.2 , with an ontogenetic increase in osmoregulatory capacity. Survival at ca. 10-32 was generally high (90100 %), while complete mortality occurred in all zoeal stages (except for zoea I) at 0.165.3 . By contrast, nearly 50 % of the megalopae and all juvenile crabs survived at such low salinities. The expression of Na+/K+-ATPase and the development of transporting epithelia were studied by means of immunofluorescence light microscopy (ILM) and transmission electron microscopy (TEM). In early (stage I-II) zoeae, fluorescence staining was observed along the inner epithelium of the branchiostegites, and epithelial cells showed typical features of ionocytes. In the megalopa and first juvenile crab, ionocytes and immunolabeled Na+/K+-ATPase were located in the filaments of the most posterior gills, while no immunodetection occurred in the anterior gills. Comparison of histological and physiological results shows a close relationship between the ontogeny of osmoregulation and the expression of Na+/K+-ATPase within the transporting epithelia of the branchial chamber. In conclusion, the adult pattern of osmoregulation develops in E. sinensis through two molts, (1) from a moderately hyper-iso-regulating zoeal phase to the moderately hyper-/hypo-regulating megalopa, (2) from the megalopa to a strongly euryhaline, hyper-/hypo-regulating first juvenile crab stage. The results of this study are consistent with an export strategy in this holo-euryhaline crab species.