Activity of acetylcholinesterase (AChE) of the pine wood nematode, Bursaphelenchus xylophilus was examined by applying the radiometric method of JOHNSON and RUSSEL (1975). The optimum pH and temperature of the enzyme were about 7.0 and 35°C, respectively. The values of the two kinetic parameters evaluated for this enzyme were 6.25μ M for the Michaelis constant (K), and 8.7 nmole/min/mg protein for the maximum velocity (Vmax) at pH 7.0 and 25°C. The fraction which showed the highest specific activity was the pellet obtained after centrifugation at 105, 000×g for 60 min, followed by the pellet obtained after centrifugation at 20, 000×g for 30 min. The activity of this enzyme was inhibited 100 or 300 times more strongly by the oxon type of organophosphorus control agents such as fensulfothion, mesulfenphos and thionazin than the thiono type. Anthelmintic agents such as morantel tartrate and levamisole hydrochloride and carbamoyl nematicide/insecticide such as methomyl also showed a potent anti-AChE activity.