An experiment was undertaken to study the effect of sub lethal effect of phorate on the acetylcholinesterase production in the Indian Major Carp rohu Labeo rohita (Linn.). The fishes were exposed to sub-lethal concentration of phorate at a dose of 18.846 μg/l. On 10, 20 and 30 days of exposure of sub-lethal test concentration of phorate, the acetylcholinesterase level in the liver, muscle, gill and brain of the fishes were estimated. Compared with control fish, the liver acetylcholinesterase content decreased to 37.21%, 52.41% and 66.51% respectively on 10, 20 and 30 days of exposure of phorate in the fishes. Compared with control fish, the AChE content decreased to 50.34%, 62.64% and 72.71% respectively on 10, 20 and 30 days of exposure of phorate in the fishes on comparison with control. Compared with control fish, the AChE content in gill decreased to 69.07%, 74.79% and 77.58% days respectively on 10, 20 and 30 of exposure of phorate in the fishes.