Elimination kinetics of six tetra-, penta- and hexachlorobiphenyls were investigated in Nereis diversicolor from two populations of the Weser Estuary by laboratory experiments. Elimination half-lives increased in the order 101, 52, 87, 44, 95, 153 (IUPAC Nos.) indicating the importance of PCB structure. At appropriate salinities, 10 ppt and 25 ppt, ragworms from the possibly more contaminated mesohaline site eliminated these components considerably faster than those from the polyhaline site. Worms from the mesohaline site also had considerably more polar transformation products of No. 52, which had been applied super(14)C-labelled. This indicates that the faster PCB elimination in this population was generally due to increased biotransformation. PCB elimination was not significantly changed after animals of the polyhaline site had been acclimated to 10 ppt salinity.Relative elimination rates of the components were in accordance with PCB patterns determined in environmental samples of Nereis diversicolor, i.e. selective elimination strongly influences the levels of PCB components. Besides No. 101, hexa- and heptachlorobiphenyls with Cl substitution in 2,3,4,5-position of one phenyl ring were under-represented. Tetra- and pentachlorobiphenyls predominated in ragworms of the mesohaline site, while hexa- and heptachlorobiphenyls did so in those of the polyhaline site. A characteristic biotransformation of PCBs is suggested to occur in the species.