EROD activities in the digestive gland of Manila clams were assayed in an attempt to select an early warning sistress signal of marine environmental degradation in the intertidal zone. The experiment showed a dose-response relationship for the EROD induction in Manila clams over wide concentration ranges of injected β-naphthoflavone. The increase of EROD activity was the highest in the digestive gland among different organs of Manila clam. EROD activities assayed in Manila clams collected at different sites along the western coast of Korea showed a significant geographical difference. Suitability of EROD activity in the digestive gland of Manila clam as an early warning distress signal are discussed. of EROD activity was the highest