We studied the presence of zinc tolerance in enchytraeid worm (Cognettia sphagnetorum, Oligochaeta) from a metal-polluted forest soil in The Netherlands. In a dose response experiment, we compared Zn sensitivity, measured as body growth and reproduction, of these enchytraeids with that of animals taken from three unpolluted sites. Because C. sphagnetorum is a keystone species, regulating microbial processes in coniferous forest soil, we performed a microcosm experiment to study the interaction of enchytraeids from several sites with soil microbes. The idea was to study whether there is a link between metal stress response of individuals (tolerance level, life history alteration) and changes observed at higher organization levels of the biological system (trophic interaction and decomposition processes). We did not find evidence for increased metal tolerance of C. sphagnetorum. Worms from the polluted site actually had reduced body growth, indicating negative fitness effects caused by long-lasting metal stress. The density and biomass of the worm population from the polluted site was low in Zn contaminated soil. Presence of enchytraeids enhanced and Zn contamination reduced the activity of microbes in the soil. Enchytraeids from different sites with different life histories and population development, however, had the same effect on microbes. Hence, observed stress responses of individuals and populations could not be linked to density-dependent trophic interactions and ecosystem functioning in the soil-decomposer food chain.