Amperometric biosensor based on enzyme acetylcholinesterase (AChE; EC 3.1.1.7) was tested for pesticide methamidophos assay. Biosensor consists from four screen printed platinum electrodes on ceramic strip. AChE was physically adsorbed onto the electrode surface. The measuring principle was based on the inhibition of AChE activity in the presence of methamidophos. The proposed method limit of detection was 2.45 nM, responding to 3.46 pg of methamidophos detected absolutely when we consider the sample volume.