Polybrominated dibenzo-p-dioxins (PBDDs) are known to be formed as byproducts in connection with the manufacture and combustion of products containing brominated flame retardants. However, to date little is known about the occurrence of PBDDs in biological samples. The aim of the present investigation was to examine the presence of PBDDs in blue mussels (Mytilus edulis) from the Baltic Sea employing a procedure adapted for dioxin analysis. Two triBDDs (1,3,7-triBDD and 1,3,8-triBDD) were identified in biota here for the first time. This identification was based on accurate mass determination and comparison of retention times on three gas chromatographic columns of different polarities (PTE 5, SP-2331, and OV1701/heptakis) with synthesized standards, together with comparisons of electron capture negative ionization (ECNI) and electron ionization (EI) mass spectra. In addition, five PBDDs and one polybrominated dibenzofuran (PBDF) were tentatively identified; altogether, one diBDD, three triBDDs, three tetraBDDs, and one triBDFwere detected in the blue mussels. To our knowledge this is the first time PBDDs have been identified in biota of the Baltic Sea. The sigma triBDD concentration in the blue mussels was estimated to be 170 ng/g lipids. The origin of these PBDDs remains unclear, but a plausible hypothesis could be biogenic formation in the marine environment.