An impedimetric biosensor to determine the activity of phospholipase A2 has been developed for the first time. This biosensor is based on the immobilization of the enzyme phospholipase A2 (PLA2) from snake venom on the surface of a gold electrode. The enzymatic reaction takes place in the presence of the L-α -phosphatidylcholine (lecithin) substrate, in 10 mM pH 7.4 phosphate buffer. The impedance of the enzymatically modified electrode/ electrolyte interface increased as the concentration of lecithin increases. A detection limit of 10−11 M and a linear range of 10−10 M to 10−5 M were obtained. At a fixed concentration of lecithin (10−3 mM), when the PLA2 enzyme with a certain level of activity is injected, the impedance of the enzymatically modified electrode/ electrolyte interface changed in the inverse direction, due to the competitive effect of the injected PLA2 enzyme. A linear range of 10 to 400 ng/L (corresponding to 16 to 640 U/L) was obtained for commercial naja mossambica mossambica snake venom. The activity of PLA2 from naja cobra snake venom (Origin Thailand) was determined to be 75% of that of naja mossambica mossambica snake venom.