The benthic diversity of the Rhine-Meuse estuary has been investigated by means of three diversity indices: the Shannon-Weaver information function, an index obtained by SANDERS' rarefaction method and the number of the species per sample. Succession proved to be a very important factor in the realization of diversity. Also the median grain-size of the sediment proved to be correlated significantly with diversity, sandy sediments containing more diverse communities than muddy ones. When succession and median grain-size are included in the analysis, salinity, nevertheless, proves to be correlated very significantly with diversity. The low diversity of brackish ecosystems also appears from the relatively high number of introduced species in these habitats.