A new highly sensitive amperometric method for the detection of organophosphorus compounds has been developed. The method is based on a ferophthalocyanine chemically modified carbon paste electrode coupled with acetylcholinesterase and choline oxidase co-immobilized onto the surface of a dialysis membrane. The activity of cholinesterase is non-competitively inhibited in the presence of pesticides. The highest sensitivity to inhibitors was found for a membrane containing low enzyme loading and this was subsequently used for the construction of an amperometric biosensor for pesticides. Analyses were done using acetylcholine as substrate; choline produced by hydrolysis in the enzymatic layer was oxidized by choline-oxidase and subsequently H₂O₂ produced was electrochemically detected at +0.35 V vs. Ag/AgCl. The decrease of substrate steady-state current caused by the addition of pesticide was used for evaluation. With this approach, up to 10⁻¹⁰ M of paraoxon and carbofuran can be detected.