The rate of uptake of 1, 2, 3, 4, 10, 10-hexachloro-6, 7-epoxy-I, 4, 4a, 5, 6, 7, 8, 8a-octahydro-1, 4-endo, exo-5, 8-dimethanonaphthalene (a component of dieldrin abbreviated HEOD) from contaminated water or food by the reticulate sculpin (Cottus perplexus) and fish-food organisms, tubificid worms (Tubifex sp.) and midge larvae (Chironomidae) was determined. Fish were held in aquaria with continuously renewed water that had concentrations of HEOD ranging from 0.017 to 8.6 parts per billion (ppb). Accumulation of HEOD by the fish was dependent on its concentration in the water, and the accumulation was nearly constant at each exposure during the 32-day test period. The saturation of HEOD in the fish apparently was never reached. Feeding experiments, using contaminated tubificid worms as the food source, showed that the retention of HEOD by fish was inversely related to the amount of HEOD they consumed. Fish retained nearly all of the HEOD they consumed when given small amounts in their food, but they retained a much lower proportion of the available HEOD when given large amounts in food. Feeding duration (up to 30 days) of the fish had little effect upon their retention of HEOD. It is evident that fish can accumulate HEOD both from the surrounding water and from their food, but accumulation of HEOD from the water does not appear to be additive with the HEOD accumulated from the food. Fish held in water having a concentration of 0.5 ppb HEOD, and fed worms containing known amounts of HEOD, did not accumulate more HEOD than fish held in the same concentration of HEOD and fed uncontaminated worms. Many examples of organochlorine insecticides contaminating our environment have been documented. Experimental laboratory studies show that aquatic animals rapidly accumulate insecticides directly from the water, probably through the skin or gills (Gakstatter and Weiss 1967, Reinert 1967, Ferguson et al. 1966). Results from studies in which field-collected samples of wildlife were analyzed for insecticides do not always point to the same conclusions. Some of the studies indicate that insecticide concentrations increase as the insecticides are passed along the food chain (Hunt 1966, Hunt and Bischoff 1960, Woodwell et al. 1967). In other field studies, the pesticide concentrations found in the aquatic organisms show no clear relation to the trophic 1 A contribution from the Pacific Cooperative Water Pollution Laboratories, Oregon State University. Technical Paper 2596, Oregon Agricultural Experiment Station. 2 Present address, Department of Animal Physiology, University of California-Davis, Davis. level ranking of the organisms, that is, their position in a food chain (Keith 1966, Peterle 1966). The transfer of pesticides from one organism to another in the aquatic ecosystem is not well understood. A knowledge of the processes involved in the transfer of insecticides into and out of living organisms and in the accumulation and e!imination of insecticides by the organisms would contribute to our understanding of the cycling of these insecticides in the ecosystem. The problems of studying pesticide distribution and movement in a natural aquatic environment are many, and only by studying these problems through laboratory experiments, under controlled conditions, can we hope to gain the fundamental knowledge necessary for understanding and evaluating the ecological changes that pesticides produce. The objective of the present investigation was to determine the rates of accumulation of dieldrin from water and food by selected fish and fish-food organisms under closely