Abstract: When the giant freshwater prawn, Macrobrachium rosenbergii is exposed to organophosphorous pesticides like Quinalphos and Dimethoate, the pesticides react with acetylcholinesterase present in nerve terminals changing it into an unreactive compound. This leads to accumulation of acetylcholinesterase in synaptic terminals which causes many neurological manifestations like irritability, restlessness, muscular twitching, convulsions or may even cause the death of the animal.