Abstract 1. Question Marine ecosystems are facing vastly increasing amounts of anthropogenic pollution, although not all pollutants posing a current threat have been recognized. Since World War I excess munition was dumped in coastal waters and for a long period of time, the dumped munition was not seen as a concern. But the metall shells are slowly corroding away and toxic compounds such as 2,4,6- trinitrotoluene (TNT) leak into the environment. For endemic organisms these chemicals represent a challenge, especially with respect to their cellular defence mechanisms. One crucial cytoprotective mechanism is the oxidative stress response. TNT was shown to lead to accumulated reactive oxygen species (ROS), hinting at its capability to unhinge a cell’s redox-balance. This lead to the question if the exposure to TNT effects the gene expression of key oxidative stress response genes (biomarkers) in the marine keystone species Mytilus spp. 2. Methods Mussels were exposed to TNT in acute and chronic lab experiments as well as in field studies at a munition dumpsite in the Baltic Sea. mRNA was extracted from the mussels’ gills and hepatopancreas. The gene expression of oxidative stress biomarkers was measured with Real-Time PCR and quantified using the relative quantification method 2-∆∆CT. The respective coding gene sequences of the selected biomarkers nrf2, nf-ᴋb, hsp90 and bcl-2 were bioinformatically predicted and identified in Mytilus spp. 3. Results In mussels in controlled as well as natural settings the exposure to sublethal TNT concentrations does have significant effects on the regulation of the target genes. TNT induced initially a significant upregulation of both transcription factors Nrf2 and NF-ᴋB in the gills and the hepatopancreas. After long-term exposure, in the gills a slight reduction was found, whereas in the hepatopancreas both TFs remain highly upregulated. The anti-apoptotic protein Bcl-2 was also upregulated at short-term exposure but interestingly after long-term exposure significantly decreased. Overall, the regulation was found to be tissue specific and dependent on the length of exposure. 4. Conclusion These findings highlight the complexity of the interaction of TNT within the oxidative stress response system. On the one hand, TNT can lead to a long lasting imbalanced cellular redox state that severely impacts the organism’s health. On the other hand, oxidative stress biomarkers could be further developed as early warning system for TNT contaminated munition dumping sites.