An amperometric biosensor based on the immobilization of organophosphorus hydrolase (OPH) onto screen-printed carbon electrodes is shown useful for the rapid, sensitive, and low-cost detection of organophosphate (OP) nerve agents. The sensor relies upon the sensitive and rapid anodic detection of the enzymatically generated p-nitrophenol product at the OPH/Nafion layer immobilized onto the thick-film electrode in the presence of the OP substrate. The amperometric signals are linearly proportional to the concentration of the hydrolyzed paraoxon and methyl parathion substrates up to 40 and 5 μM, showing detection limits of 9 × 10(-)(8) and 7 × 10(-)(8) M, respectively. Such detection limits are substantially lower compared to the (2-5) × 10(-)(6) M values reported for OPH-based potentiometric and fiber-optic devices. The high sensitivity is coupled to a faster and simplified operation, and the sensor manifests a selective response compared to analogous enzyme inhibition biosensors. The applicability to river water sampling is illustrated. The attractive performance and greatly simplified operation holds great promise for on-site monitoring of OP pesticides.