Monochamus alternatus Hope is a main insect vector of pinewood nematodes (Bursaphelenchus xylophilus Nickle), which causes the serious pine wilt disease. Organophosphate is a kind of common insecticide used for the control of the pest insect. In birds and mammals, some organophosphates can cause a syndrome called organophosphate-induced delayed neurotoxicity by inhibiting neurotoxic esterase (NTE). The activity of the enzyme was found in various organisms, but people know little about it in insects and especially its physiological functions at the present time. We detected the NTE activity in larva of M. alternatus by using the differential assay method, and found that the enzyme activity in the head was 2.80±0.30 nmol·min^-1·mg^-1, which was nearly 10 times that in the fat body of the insect, but much lower than that in birds and mammals. The NTE activity from the insect head could be inhibited by methamidophos in vivo; however, in vitro, the NTE activity was found to be insensitive to methamidophos, which was quite different from that in birds and mammals.