Bioconcentration factors (BCF) or bioaccumulation factors (BAF) reported for 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD or dioxin) in aquatic environments encompass a wide range of values, from less than 1000 to 189,000 I/kg. These values are based on concentrations of TCDD in various environmental media including water, sediment, or food. Under the Federal Water Pollution Control Act and its enabling regulations (40 CFR 700–740, 400–470), point source discharge limits are established so that the nominal receiving water concentration will not exceed the water quality criterion. To be consistent with this regulatory process, the water quality criterion should also be calculated using an accumulation factor that is based on a nominal water concentration. The regulatory process for developing a water quality criterion for TCDD requires the selection of a BAF that describes the relationship between the source to be regulated and the fish tissue concentration.