This chapter discusses electrophoretic studies, acetylcholinesterase activity, optomotor behavioral changes, histopathology with a light microscope, magnified structures with scanning, and ultrastructures with transmission electron microscopy. It also discusses two important pesticides, those are Malathion and diazinon. Malathion and diazinon are also used as insecticides in paddy fields. They pollute the aquatic environment by direct application, spray drift, aerial spraying and washing from the atmosphere by precipitation, erosion and runoff from agricultural land, and by discharge of effluent, from factories and in sewage. Malathion has a low toxicity for mammals and a relatively high toxicity for fish. This happens due to the lack of hydrolytic enzymes in insects and fish. Oxygen analogues of malathion appear to be the active part that binds vigorously to acetylcholinesterase. This malaoxon is hydrolyzed rapidly in mammals thus becoming inactive, but such hydrolysis does not occur in insects and proceeds very slowly in fish. Diazinon is a broad-spectrum insecticide.