A novel method for immobilization of acetylcholinesterase (AChE) by binding covalently to a cross-linked chitosan-multiwall carbon nanotube (MWNT) composite is described. In addition a sensitive, fast, cheap and automatizable flow injection detection of an organophosphorous insecticide was developed. The MWNTs were homogeneously distributed in the chitosan membrane which showed a homogeneous porous structure. The immobilized AChE could catalyze the hydrolysis of acetylthiocholine with a K(M)app value of 177 microM to form thiocholine, which was then oxidized to produce detectable signal in a linear range of 1.0-500 microM and fast response. MWNTs could catalyze the electrooxidation of thiocholine, thus increasing detection sensitivity. Based on the inhibition of an organophosphorous insecticide on the enzymatic activity of AChE, using Sulfotep as a model compound, the conditions for the flow-injection detection of the insecticide were optimized. Both biocompatibility of chitosan and inherent conductive properties of MWNTs favored the detection of the insecticide from 1.5 to 80 microM along with good stability and reproducibility. 95 % reactivation from inhibited AChE could be regenerated by using 2-pyridinealdoxime methiodide within 15 min for 15 times. The detection of Sulfotep samples exhibited satisfactory results. The proposed flow-injection analysis device can be applied to automated determination and characterization of enzyme inhibitors.