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The substrate specificity of brain acetylcholinesterase (AChE) in adult Carassius auratus fish and its sensitivity to carbamate insecticides were investigated in vitro. The results showed that the order of four substrates hydrolyzed by brain AChE in C. auratus was acetylthiocholine iodide > β-methylthiocholine iodide > propionylthiocholine iodide > butyrylthiocholine iodide, and the maximum velocity (Vmax) of AChE hydrolyzing acetylthiocholine iodide (ATCh) was the highest among the four substrates, and the Vmax values were 0.067 and 0.082 mmol min−1 mg−1 for male and female fish respectively. But their Michaelis-Menten constants (Km) were the lowest, only 0.071 and 0.072 mmol/l respectively. Compared with other carbamate insecticides, the sensitivity of brain AChE to carbofuran was the highest and the IC50 values were 1.04 × 10−6 mol/l for females and 1.17 × 10−6 mol/l for males. The inhibitory tendencies of eserine, methomyl, and aldicarb to brain AChE were very similar, and the percentage inhibition increased with time at the concentration of 1 × 10−6 mol/l. The order of inhibition potential of the three inhibitors from the highest to the lowest was eserine, aldicarb, and methomyl.