The marine environment is under continuous stress due to various types of contaminants. Since the water is most polluted in the coastal side, it has an adverse effect on organisms like fishes, oysters, etc. We take in those fishes which already are contaminated with deposition of metals or other toxicants resulting in various abnormalities. To detect the presence of toxic pollutants various methods are there out of which biomarker is one of them. Acetyl cholinesterase or AChE hydrolyses acetylcholine during motor impulses in our body. Neurotoxic contaminants blocks the active sites resulting in the reduced AChE activity leading to neurological disorders. The work in this book presents an evaluation of the acetyl cholinesterase activities of Nerita chamaeleon and Etroplus suratensis. This book should be useful to the students and professionals working in the field of ecotoxicology, toxicology, neurology and chemists.