Carps (Cyprinus carpio) and red swamp crayfish (Procambarus clarkii) were sampled from two lowstream Mediterranean rivers (Anoia and Cardener) receiving extensive urban and industrial waste water discharges. Tissue residues of selected pollutants (organochlorinated compounds, polycyclic aromatic hydrocarbons (PAHs)) were determined in conjunction with different biochemical responses (cytochrome P450, phase II enzymes) with the aim of investigating whether resident organisms were responsive to changes in water quality. Biota inhabiting those rivers were highly exposed to complex mixtures of polychlorobiphenyls and dichlorodiphenyltrichloroethanes (up to 19 ng/g w.w.) and PAHs (up to 6097 ng/g of hydroxylated PAHs in bile), the highest residues being observed in carps from Cardener. This has a reflection on 7-ethoxyresorufin O-deethylase (EROD) activity; that in carps from Cardener ranged between 350 and 550 pmol/min/mg protein, whereas in carps from Anoia ranged between was 90 and 250 pmol/min/mg protein. The highest activity recorded was downstream of the sewage treatment plants in both rivers. Phase II enzymes were less sensitive to polutant exposure than EROD; nonetheless, both glutathione S-transferase and UDP-glucuronyl transferase were higher in carps from Cardener. The results support the usefulness of the combined use of chemical and biochemical responses to assess and diagnose pollution in highly stressed ecosystems.