Malathion, a widely used insecticide, is known to cause serious metabolic disturbance in non- target organisms, like fish. Bioassay tests were conducted on the grass carp fish to evaluate the acute toxicity of malathion by determining their LC50 after 24, 48, 72 and 96 h exposure. The safe concentration of this biocide was calculated on the basis of LC50 for 96h. The fish showed characteristic changes in behavior when exposed to various concentrations of malathion with difficulty in respiration, convulsions and short erratic jerky body actions. The sublethal toxic potency of malathion was
indicated by inhibiting acetyl cholinesterase activity in the brain, muscle, gill and liver tissues of the grass carp. The changes in acetyl cholinesterase activity in the above tissues were decreased significantly up to 96h. Maximal inhibition
of the enzyme was observed at 24 and 48 h intervals. So, by acquiring knowledge of LC50 values of a toxicant, its discharge into nearby water resources may be regulated to protect aquatic life.