The population structure of Assiminea grayana Fleming, 1828 (Gastropoda, Assimineidae), in the South-West Netherlands

— Population structure and life-cycle of Assiminea grayana are described, based on size frequencydistribution histograms of samples from a salt-marsh along the Western Scheldt estuary. — Generations of previous years could be distinguished until June, later on they merged to form one inseparable group. — Highest numbers were found in winter, numbers decreased at the end of the winter, mainly due to mortality, and in spring, mainly due to migration. — First representatives of the new generation were found in July. In October the main part of the population consisted of this new generation. — At least part of the individuals may live for two years.