An organophosphorus (OP) pesticide sensor was fabricated by applying a pH-sensitive polymer coating and organophosphorus hydrolase (OPH) enzyme onto the surface of a magnetoelastic sensor, the magnetic analogue of the better-known surface acoustic wave sensor. Organophosphorus hydrolase catalyses the hydrolysis of a wide range of organophosphorus compounds, which changes the pH in the hydrogel. This article describes the application of the magnetoelastic sensor for the detection of OP pesticides by measuring the changes in viscoelasticity caused by the swelling/shrinking of the pH-responsive polymer when exposed to the pesticides. The sensor was successfully used to detect paraoxon and parathion down to a concentration of 1 x 10(-7) and 8.5 x 10(-7) M respectively.