Although the uptake, storage, and elimination of contaminants by naiad mollusks has been studied, relatively little information is available on toxicity. Contaminants appear to have destroyed some populations directly by exerting toxic effects, or indirectly by causing or contributing to the elimination of essential food organisms or host fish. Often two or more factors appear to work in combination to producethe total stress that adversel affects populations. Naiad mollusks are important indicators of contaminants in the environment; residues in soft tissue indicate recent or current exposure, and residues in shells indicate past exposure.