The Chinese mitten crab (Eriocheir sinensis) was introduced into northern Germany at the beginning of our century. Sexually mature crabs migrate downstream in order to mate in estuarine regions. The larvae hatch in spring in the lower parts of river mouths and in the Wadden Sea, where the first part of their development takes place. Most of the spawned crabs will die in the Wadden Sea. New experimental observations on the salinity tolerance of the larvae show that complete development is not possible in rivers or in brackish estuarine conditions. The five successive zoeal stages reveal a decreasing tolerance to low salinity, in particular when water temperature is also low. This suggests that a major part of larval development takes place in the open sea. The ultimate larval stage, the megalopa, is more tolerant to brackish water than the zoeae. Thus, it shows the ability (but not the necessity!) to immigrate into estuaries.