A flow-through system based on acetylcholineesterase (AChE) was studied. The system was prepared by mixing AChE and a multiwall carbon nanotube (MWCNT). Two important parameters, the ratios of AChE:MWCNT ($X_1$) and AChE-MWCNT:sol-gel ($X_2$) were optimized using response surface methodology. The results revealed that an enzyme immobilized within the MWCNT-sol-gel was more effective compared to one conducted with sol-gel. The optimum feed flow rate was 0.4 mL/min and ATChI concentration was found to be 1 mM. The optimum ratios of $X_1$ and $X_2$ for immobilization on ceramic packing were 1.07 and 0.43, respectively. The sensitivity of this flow-through system was $1.82{ imes}10^{-5}/{mu}M$ and long-term stability analyzed after 120 days was 74% of initial absorbance. With respect to an incubation time of 14 min, the detection limit for paraoxon was $7.3{ imes}10^{-12}$ mol.