Samples, collected from 1959 to 1970, were used to describe the distribution and ecology of some oligochaetes in the Delta region. In the brackish and marine parts of the estuary the following species were found;Tubifex costatus, T. pseudogaster, Peloscolex benedeni and Monopylephorus rubroniveus. The other identified species were more or less euryhaline-freshwater organisms. The distribution pattern of these species was correlated with salinity and sediment. The species had a wider chlorinity tolerance than reported in the literature. The preference of Peloscolex benedeni for clay sediments was due to low oxygen or hydrogensulfide conditions in the sediment. Limnodrilus hoffmeisteri preferred well sorted muddy sand bottoms and L. udekemianus less well sorted clay bottoms. Tubifex costatus occurred abundantly in intertidal regions. The occurrence of most of the collected oligochaetes was positively correlated with the quantity of organic matter in the sediment. An investigation carried out from 1971 to 1977 to study the development of the macrofaunal community in the fresh Haringvliet-Hollands Diep basin after closure of this basin showed a mass development of Tubificidae. This was related to the high organic matter content in the bottom and the high colonization and reproductive capacity of these worms. It appeared that several species of tubificids coexisted for several years in the same habitat at a roughly constant relative abundance. Several investigators showed a selective feeding of tubificids on certain groups of bacteria. A niche delimitation probably exists through the nature of the food used by these tubificids. A detailed account of the results presented here will be given elsewhere.