Derived polycyclic aromatic hydrocarbons (PAHs) such as nitro-PAHs are present in the environment and are known to be much more toxic than PAHs compounds. However, very few studies have analysed their effects on the aquatic environment and none have investigated the freshwater environment. In the present study, we determined whether 1-nitropyrene (1-NP), a model of nitro-PAHs, can induce DNA adducts in gills and digestive glands of the freshwater mussel Dreissena polymorpha. Two concentrations of 1-NP (50 and 500 mu M) were tested. In addition, in order to understand the metabolic pathways involved in 1-NP genotoxicity, mRNA expression of genes implicated in biotransformation mechanisms was assessed by quantitative reverse transcription-PCR. Results showed the presence of DNA adducts in both gills and digestive glands, with highest levels obtained after 5 days of exposure to 500 mu M. Metallothionein mRNA levels were enhanced in digestive glands exposed to 50 mu M. Surprisingly, at the higher concentration (500 mu M), aryl hydrocarbon receptor and HSP70 genes were only up-regulated in digestive glands while PgP mRNA levels were increased in both tissues. Results suggested a cytotoxic and genotoxic effect of 1-NP. Mussels seemed to be able to partially detoxify this compound, in view of the low amount of DNA adducts observed after 5 days exposure to 50 mu M. For the first time, 1-NP biotransformation and detoxification systems have been characterised in D. polymorpha.