Tetrabromobisphenol A (TBBPA) and hexabromocy-clododecane diastereoisomers (α-, β-, and γ-HBCD) were investigated in effluents from sewage treatment works, landfill leachates, sediments, and food web organisms of the North Sea basin. Residues were quantified by liquid chromatography-mass spectrometry. Both flame retardants were enriched in sewage sludges, where a maximum total (Σ) HBCD concentration of 9.1 mg/kg (dry weight; d.w.) was found; TBBPA was at levels of 102 μg/kg. Landfill leachates from The Netherlands showed up to 36 mg (ΣHBCD)/ kg (d.w.). γ-HBCD dominated isomeric profiles in sediments, and concentrations were elevated near to a site of HBCD manufacture. α-HBCD was the primary congener detected in marine mammals; however, very few samples exhibited TBBPA. ΣHBCD ranged from 2.1 to 6.8 mg/kg (lipid weight; l.w.) in liver and blubber of harbor porpoises (Phocoena phocoena) and seals (Phoca vitulina). TBBPA levels in cormorant (Phalacmcorax carbo) livers were up to 1 order of magnitude lower compared to ΣHBCD. HBCD in eels (Anguilla anguilla) from the Scheldt basin (Belgium) reflected the spatial distribution of concentrations in local sediments. This study shows evidence of HBCD bioaccumulation at the trophic level and biomagnification in the ascending aquatic food chain, and these findings justify risk assessment studies at the ecosystem level.