The expanding rice crop area and the changing from traditional long life-span single crop to shorter life-span triple annual crops in the Mekong delta may stress aquatic life, including physical disturbances and the use of toxic agrochemicals. Here, we exposed climbing perch (Anabas testudineus) to diazinon (1, 2 and 10%LC50-96h) in the laboratory and to the field condition to see the responses of their cholinesterase activity. Results shown that Diazinon caused long-term brain acetylcholinesterase (AChE) inhibition in both laboratory and field exposures. Enzyme activity in this species is very sensitive to diazinon. AChE was significantly inhibited by only 66 mg/L in the laboratory and prolong inhibition after a single field application. These findings suggest that assessment of brain AChE of this species could serve as a biomarker of pesticide exposure in the Delta region.