This study was designed as a part of efforts to investigate the biochemical pollutant markers for diagnosis of marine pollutions by changes in cholinesterase activity of the mussel (Mytilus coruscus) in South Sea of Korea. Acetylcholinesterase (AChE) activities in muscle of cultured mussels in South Sea were remarkably lower (20∼41%, respectively) than those of wild mussel in Pohang(control) of East Sea. Acetylcholine (Ach) activities in muscle of cultured mussels in South Sea were remarkably lower (15∼30%, respectively) than those of wild mussel in Pohang of East Sea. Monoamineoxidase (MAO-B) activities in muscle of cultured mussels in South Sea were significantly 2∼19% higher than those of wild mussel in Pohang of East Sea. It suggests that AChE, ACh and MAO-B activities in muscle of cultured mussels of South Sea may be used as the most effective mean in a biochemical markers for early warning of environmental damages caused by organophosphorus pesticides.