Abstract A sensitive screen-printed amperometric sensor suitable for rapid determination of the concentration of the insecticide methamidophos was developed. It was based on the principle of inhibition of acetylcholinesterase (AChE) activity. The first part of the study was focused on the screening of several genetically modified AChEs in order to select the most sensitive enzyme towards the methamidophos. Values for the bimolecular constant k i were also determined. In a second part of the study, we compared the lowest detectable methamidophos concentrations using different immobilised AChEs. The lowest detectable concentration in a standard solution was 1.4 ppb for the AChE(Dros)-B03 mutant compared to 4.8 ppb for the wild type AChE and 53 ppb for the electric eel source.