HUMAN activities have greatly increased the flux of many potentially toxic metals to aquatic ecosystems(1). The development and implementation of effective remedial measures depend on our ability to predict the fate and effects of metals in these systems. Models based on sound physical-chemical and biological principles, such as the free-ion activity model(2-5), have shown great potential as predictive tools. This model has been effective in explaining the central role of the free-ion concentration (or activity) as a regulator of interactions (uptake, toxicity) between metals and aquatic organisms(2,3). It postulates that the biological effects of metals are best predicted by the activity of the free metal ion, rather than by total metal concentration. Because this model was developed in the laboratory under unnatural experimental conditions, it must be validated in field situations before being generally used in nature(3). We report here that Cd concentrations in an indigenous aquatic insect larva, Chaoborus punctipennis, are best described by the free-ion activity model, provided that competition for biological uptake sites between hydrogen ions and free cadmium ions, as well as cadmium complexation by natural organic matter, are explicitly taken into account. Our results suggest that the free-ion model would provide an effective theoretical framework for the use of animals as indicators of metal contamination in nature.