Commercial formulations of some organophosphate pesticides caused rapid inhibition of enzyme electrode function. The aim of this study was to develop a new enzyme electrode for determination of organophosphorus pesticides (OP). The enzyme electrode is based on polyprrole (PPy) composite and the enzyme acetylcholinesterase (AChE). The (PPy-AChE) modified electrode was utilized for the sensitive electrochemical detection of thiocholine at the gold surface after hydrolysis of acetylthiocholine (ATCh) by the immobilized enzyme. The enzyme electrode operates at a potential of +200 mV versus Ag/AgCl in a pH buffered solution with 0.1 M phosphate (0.1 M KCl). Inhibition by paraoxon required a period of pre-incubation in the pesticide before addition of enzyme substrate. The decrease of substrate steady-state current caused by the addition of pesticide was used for evaluation. The influence of several experimental parameters such as applied potential, solution pH value, incubation time was explored to optimize the electro analytical performance of the enzyme electrode. Under the optimal conditions the enzyme electrode was used to measure as low as 0.2 mu M paraoxon with sensitivity of 1.77 nA/mu M. An inhibition calibration curve was obtained using paraoxon, an inhibitor of acetylcholinesterase. The enzyme electrode is sensitive, small, and inexpensive and can be operated by non-professional technicians.