An explanation is found for the lack of bioaccumulation of a great number of polychlorinated dibenzo-p-dioxin (PCDD) congeners. Two groups of goldfish were exposed to a fly-ash extract, which was dissolved in water with the aid of a generator column. The first group was exposed to piperonylbutoxide (PBO), which inhibits monooxygenase activity. Congeners which lack chlorine substitution on the lateral positions (2,3,7,8-positions) or which have one or more pairs of unsubstituted vicinal places are found in these fish. In the second group of fish, however, only 2,3,7,8-TCDD was found after 6 days. Therefore all other congeners are likely to be biotransformed rapidly in the fish which were not treated with PBO.