The recent widespread production and use of flame retardants, polybrominated diphenyl ethers (PBDEs), is one of the reason of the increasing contamination observed worldwide. At the present, deca-BDE mixture, in which the decabromodiphenyl ether (BDE-209) is the major congener (98%), dominates the EU market. The potential genotoxicity of BDE-209 was examined in the freshwater bivalve zebra mussel (Dreissena polymorpha) by means of Comet assay and micronucleus assay (MN assay). Mussels were exposed in vivo to BDE-209 at nominal concentration of 0.1, 2, and 10 μg/l under laboratory conditions. The assays were performed on the bivalve hemocytes monitoring the levels of DNA strand breaks and the percentage of micronuclei until 168 and 96 hr of exposure, respectively. At the same time, BDE-209 levels were measured daily in mussel soft tissues to evaluate the bioaccumulation. Results of the Comet assay showed a significant increase of DNA damages compared to controls, but a lack of dose/effect relationship probably due to the formation of less-brominated congeners. By contrast, no significant changes in MN frequency from baseline levels were observed. These preliminary results about the potential genotoxicity of this compound in invertebrates indicated a clear BDE-209 capability to induce DNA damage, but no irreversible effects on DNA hemocytes. Furthermore, bioaccumulation of this high-molecular-weight substance and its uptake mechanism in zebra mussel are also discussed. Environ. Mol. Mutagen., 2007. © 2007 Wiley-Liss, Inc.