Cadmium, lead, zinc and copper concentrations were determined in 15 species of freshwater macro-invertebrates collected from a littoral foodweb. Indications of a biomagnification process, in which predators had a higher concentration than their prey, were only found in the case of zinc. Organisms with copper as an essential component of their haemolymph, such as Crustacaea, Gastropoda and Bivalvia, contained higher concentrations of that metal than other species. Another important factor in the determination of trace metal concentrations, apart from physiological equipment, was body weight, resulting in higher concentrations in smaller, and lower concentrations in larger, organisms. This seemed to be the case within populations of single species, as well as in different species of deposit feeders. Furthermore, feeding habit, proximity to the sediment and physico-chemical factors appeared to be determining factors for trace metal concentrations in macro-invertebrates. The results of this study are related to ecotoxicological theories which explain the causes of pollutant residues in organisms. It is concluded that various explanations for the differences in pollutant residues, e.g. trophic level, feeding habit, body weight, association with the substrate, physiological equipment and abiotic factors, are valid.