The use of DNA adduct analysis has previously focused on the use of marine organisms for biomonitoring, whereas similar investigations in freshwater organisms are sparse. In that context, we have investigated the relevance of DNA adducts as biomarkers of genotoxicity in the freshwater mussels Dreissena polymorpha. The objective of the present study is to determine the stability of DNA adducts induced by benzo[a]pyrene (B[a]P) in zebra mussels. Mussels were exposed to dissolved B[a]P (10100 mu g/l) for 4 days. Afterwards, mussels were kept in clean water for 28 days and DNA adduct levels were subsequently measured in two different organs, the digestive glands and the gills, using the P-32-postlabelling technique. In parallel, the expression of genes involved in the detoxification system was assessed by qPCR (catalase, superoxide dismutase, glutathione S transferase, HSP70, aryl hydrocarbon receptor, P glycoprotein). We observed a higher level of DNA adducts in the digestive glands compared to the gills. Moreover, in gills, the level of DNA adduct was dependent on the B[a]P concentration. The levels of adducts tended to decrease in both organs after 28 days in clean water. In addition, an early induction of HSP70, PgP, AHR and SOD mRNA levels was noticed in the gills compared to the digestive glands. CAT and GST gene expression increased from 12h exposure in both organs. A higher gene expression level of those genes was observed in the gills, except for AHR and CAT genes. Data converge towards the fact that DNA adducts hence represent a very promising biomarker of B[a]P contamination and potentially of exposure to polycyclic aromatic hydrocarbons. In addition, for the first time in this study, B[a]P detoxification system was characterised in D. polymorpha.