Abstract: For the purpose of detecting of induction of acetylcholinesterase (AChE, EC 3.1.1.7) by organophoshorous insecticides, amount of brain AChE of a freshwater fish, i.e., topmouth gudgeon , Pseudorasbora parva, which underwent exposure of triazophos, was detected using an indirect non-competitive ELISA. The in vivo AChE was induced at such a low concentration of 0.1 μg /L. Due to the induction inhibition on activity of AChE by triazophos was masked, in some extent, by the increase of the enzyme content. Result of the present research indicated that a sensitive detecting of exposure could be achieved if specific activity of AChE was evaluated by the enzyme content, rather than by the total protein content of the crude extract.