DiscussionDespite the progress in understanding the ecological significance of parasites that we have made in recent years, we are still far away from having a thorough understanding of the ecological and ecotoxicological implications of parasites. There is an increasing number of papers showing how parasitism and pollution can interact with each other in aquatic organisms. In addition to synergistic negative effects of both stressors, there is also evidence of antagonistic interactions. The latter are related to the reduction of pollutant levels in infected hosts compared with uninfected conspecifics. As reduced contaminant concentrations are usually correlated with less adverse effects, it might be advantageous to be infected if hosts are confronted with environmental pollution. On the other hand, possible pathological effects might reduce a potentially beneficial effect of parasites. There are also examples, however, which show that parasites may enhance toxic effects of pollutants by interfering with the host's protection mechanisms. In these cases, parasites would have exclusively negative effects on the physiological homeostasis of their hosts. In the present talk selected key issues are highlighted that illustrate important contributions of parasites in the field of environmental toxicology and the implications of parasites in pollutant transfer and accumulation with subsequent effects within food-webs. By using selected examples from aquatic ecosystems, an insight into selected ecotoxicological implications of parasites will be provided.