Based upon the hypothesis that the Metazoa (nematoda and rotifera) in the interstitial stratum of a slow sand filter are of importance in the process of purification of surface water, a filter was built to check the substrate characteristics of different grain sizes compared with the natural soil. It was found that the granulometric composition of sands as found in the settling basins did have the best comparative screening properties. This was demonstrated by chemical analyses of NH4, NO2, NO3, PO4, KMnO4-consumption and O2-saturation. Only the NO3-graph is shown here. The importance of Metazoa in the process of filtering water was studied by inoculating metazoic microorganisms into the most efficient filter. Chemical analyses of the water did not reveal a significant change of water quality over that from a non-inoculated filter. Because of this result the Metazoa do not appear to act as an important component in the water purification by sand filtration.