The fate of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) was studied by using aquatic sediment and lake water under laboratory conditions. Most of the TCDD was found in the sediment from which it slowly disappeared. Evaporation may be a major mode of disappearance of TCDD in samples incubated 39 days or more, with metabolism playing only a minor role. Under the experimental conditions the half-life of TCDD was in the order of 600 days. The metabolic activities were enhanced under conditions which stimulated microbial growth in the presence of sediment, and the metabolites were found to be released from the sediment to the ambient water.