We have developed a screen-printed electrochemical electrode (SPE) for paraoxon based on its inhibitory effect on the enzyme butyrylcholinesterase (BChE). The electrode was first modified by drop casting with a dispersion of carbon black nanoparticles (CBNPs) in a dimethylformamide-water mixture, and BChE was then immobilized on the surface by cross-linking. The resulting biosensor was exposed to standard solutions of paraoxon, and the enzymatic hydrolysis of butyrylthiocholine over time was determined measuring the enzymatic product thiocholine at a working voltage of +300 mV. The enzyme inhibition is linearly related to the concentration of paraoxon up to 30 μg L-1, and the detection limit is 5 μg L-1. The biosensor is stable for up to 78...