The principal function of percolating filters used in the treatment of sewage and organic wastes is to break down organic matter, oxidizing the organic carbon to carbon dioxide. Solids are generally made more readily settleable, ammonia is frequently oxidized to nitrate, and toxic metals such as zinc and copper are partially adsorbed by the attached microbial film. The role of invertebrates present in filters in maintaining the efficiency of such systems is a complex one. It has already been demonstrated from experimental studies (Williams & Taylor 1968) that both enchytraeids and psychodid flies can prevent the blocking of filter interstices by microorganisms and sewage solids, such blocking resulting in the development of anaerobic conditions and a declining efficiency of treatment. Further experiments have been conducted (Solbe, Ripley & Tomlinson, in preparation) which demonstrate the effect of temperature on their activity and metabolism. The present study was carried out on the filter described by Solbe, Williams & Roberts (1967). The rate at which the two most abundant enchytraeids Lumbricillus rivalis and Enchytraeus coronatus consumed organic matter has been calculated from estimates of their numbers and biomass in the filter, together with laboratory measurements of their rate of consumption of oxygen. The result has been compared with the total rate of dissipation of organic matter within the filter. In determining enchytraeid densities and size distributions for biomass estimates, information on the biology of the two enchytraeid species present, particularly their lifehistories and depth distributions, has also been collected and is presented here. In the past, most faunal studies of percolating filters have been limited to surface and subsurface sampling and consequently have not adequately taken into account vertical gradients in distribution, their influence on the interaction between species and their importance in studying species requirements and life-histories.