ABSTRACT. Aclvlty of 7-ethoxyresorufin-0-deethylase (EROD) In fish is certainly the best-studied b~omarker of exposure applied In the field to evaluate biological effects of contamination in the manne environment Since 1991, a feasibility study for a monitonng network using this biornarker of exposure has been conducted along French coasts Uslng data obtalned dunng several cruises, thls study aims to determine the number of flsh requ~red to detect a glven difference between 2 mean EROD activities, I e to achieve an a prlon fixed statistical power (l-P) glven s~g n~f~c a n c e level (a),