Abstract Pulp and paper mill effluent compounds pollute the aquatic environment and are responsible for increased biochemical alterations and genotoxicity in aquatic organisms such as fish. Adult eels ( Anguilla anguilla L) were exposed during 8, 16, 24, and 72 h to the following conditions: (1) aerated, filtered, and dechlorinated tap water ( C); (2) 2.5% (v/v) sewage water previously treated with activated sludge ( T); (3) bleached kraft pulp and paper mill effluent collected at the river Vouga, close to an ancient sewage outlet (Portucel), diluted in tap water [25% ( E 25 ) and 50% ( E 50 )]; and (4) bleached kraft pulp and paper mill effluent sediment [water-soluble fraction ( S)]. Liver biotransformation induced by the above conditions was measured as ethoxyresorufin- O-deethylase (EROD), cytochrome P450 (P450) (Phase I), and glutathione- S-transferase (GST) (Phase II). Genotoxicity was also determined as blood/liver DNA strand breaks and erythrocytic nuclear abnormalities (ENA) induced on European eel ( A. anguilla L). Liver EROD activity was significantly increased in eels at 8 and 16 h exposure to E 25 , as well as at 16, 24, and 72 h exposure to E 50 . S exposure induced liver EROD activity only at 24 h. A significant decrease in liver P450 was observed at 72 h exposure to T, whereas a significant P450 increase at 16 h was followed by a significant decrease at 24 h exposure to E 25 . Another P450 significant increase was noticed at 72 h exposure to S. Liver GST activity (Phase II) demonstrated a significant increase at 72 h exposure to E 50 and to S. A significant decrease in blood DNA integrity was observed at 72 h exposure to T and at 24 and 72 h to S. Blood DNA integrity significantly decreased at 16 and 24 h exposure to E 25 , as well as at 8, 16, and 24 h exposure to E 50 . Liver DNA integrity significantly decreased at 72 h exposure to T and at 16 h exposure to S. Moreover , liver DNA integrity was significantly decreased at 24 h exposure to E 25 and E 50 , and 72 h to E 50 . A. anguilla L. increased ENA frequency was detected in T at 16, 24, and 72 h, whereas in E 25 and S it was observed at 8, 16, and 24 h. Furthermore, E 50 ENA frequency increased at 24 h exposure.