The current interest in environmental quality has stimulated research into the nature and persistence of agricultural chemicals and their metabolites. The widespread occurrence of undesirable residues from the ubiquitous use of the chlorinated hydrocarbons as pest control agents, particularly DDT, has resulted in the gradual termination of these materials as environmentally unsuitable insecticides in the United States. To insure that future pest control agents or their degradation products are not persistent and therefore undesirable, an initial screening must take place before they are put into large-scale use. The development of a model ecosystem (Metcalfet al. 1971) clearly establishes a means for evaluation of the environmental suitability of currently employed pesticides as well as those in developmental stages. As a part of a continuous program to investigate the effects of pesticides on the environment, the fate of dieldrin has been examined in a model ecosystem.