Acetylcholinesterase (AChE) was immobilized on multilayer films assembled by poly diallyldimethylammoniumrnchloride (PDDA) and ι-carrageenan (IC) on silver-coated crystal electrode surfaces to detect the chlorpyrifos belonging to thernorganophosphates pesticide. Mass sensitive quartz crystal microbalance (QCM) was used to study the effect of AChE concentrationrnand pH of phosphate buffer solution on immobilized acetylcholinesterase. The optimized conditions were as follows: pH was 6.0rnwhich was near isoelectric point (pI); enzyme concentration was 0.1 mg ? mL-1; and the lowest concentration of chlorpyrifos that couldrnbe detected was 0.1 μg ? L-1.