AbstractEnvironmental fate of TCDD was studied in an aquatic ecosystem, using an outdoor pond which had been dosed initially with 53.7 ppt of14C‐TCDD. The levels of radiocarbon reached an equilibrium in pondweeds after one month, in fish after two months and in the sediment after six months. Using the identical components from the outdoor pond, roughly comparable data were obtained in the laboratory for short‐term distribution of radiocarbon. The largest source of variance and discrepancy between the outdoor and the laboratory data was pondweeds. This material produced the largest shift in biomass due to rapid growth and subsequent massive death, causing a drastic change in distribution of radiocarbon in the second year in the outdoor ecosystem. The amounts of the total TCDD remaining in the pond after 12 and 25 months were 49.7 and 29.4%, respectively. The bulk of metabolites was detected in water and pondwe