Abstract: Using the Bac-to-Bac baculovirus expression system, the acetylcholinesterase 1 gene (ace1) of Aphis gossypii was successfully expressed in Sf9 cells. The enzyme was secreted into the cell culture medium and tested. The affinity (K_m) of the expressed AChE1 for the substrate analog ATChI was 144.5±28.6 μM. The sensitivity index (indicated by K_i value) of expressed AChE1 to 2 carbamate insecticides (pirimicarb and triazamate) and 2 organophosphate insecticides (omethoate and demeton-S-methyl) were 513, 1 883, 5 and 23 (mM·min) ^-1, respectively, which is in agreement with previously reported values obtained with the natural enzyme. The expressed enzyme showed highest activity when harvested at 2-3 d after infection of the insect cells. The expressed AChE1 was very stable without obvious loss in enzyme activity after 90 d when stored at -20 ℃, although 70% of the enzyme activity was lost after 7 d when stored at room temperature.