The activities of esterase and acetylcholinesterase(AChE) on the Diamond backmoth (DBM), Plutella xylostella (Lepidoptera: Yponomeutidae) and Beet armywarm (BAW), Spodoptera exigua (Lepidoptera: Noctuidae) and inhibitions of AChE with flupyrazofos were clarified from the results of a series of experiments. These findings are described in brief as follows. The AChE activities of DBM and BAW in heads were $1.5{sim}11.1{mu}$mol/g/min in $1st{sim}4th$ instar larvae of DBM and $1.7{sim}45.2{mu}$mol/g/min in $1st{sim}6th$ instar larvae of BAW, respectively. Those were $25{sim}30$ times higher in above 4th instar larvae of BAW than that of the 1st instar larvae of DBM. The activities of aliesterase in heads were $1.7{sim}4.7$ times higher in $2nd{sim}4th$ instar larvae of DBM and $8{sim}55$ times higher in $3rd{sim}6th$ instar larvae of BAW than 1st instar larvae of DBM. In abdomens, those were $3{sim}17$ times higher in $2nd{sim}4th$ instar larvae of DBM and $12{sim}30$ times higher in $3rd{sim}6th$ instar larvae of BAW than 1st instar larvae of DBM. Median AChE inhibition concentration $(I_{50})$ of flupyrazofos to the 2nd instar larvae of DBM and BAW were 92 nM and $15{mu}M$, respectively, and those to the 4th instar larvae of DBM and BAW were $1.8{mu}M$ and 3.1 mM, respectively. Insensitivity ratio of flupyrazofos in the 2nd instar BAW larvae showed ca. 162 times higher than that in the 2nd instar larvae of DBM, and that of the 4th instar BAW larvae showed ca. 1,720 times higher insensitivity to flupyrazofos than that of the 4th instar DBM larvae. AChE activities in the 2nd instar larvae of DBM and BAW at 32 h after applicaton of flupyrazofos decreased from 67.6% to 32.4% of the activity of the untreated control. That of the 4th instar larvae of DBM increased for 0.5 h after application flupyrazofos up to 75% of the untreated control, and after that it decreased to 34.5% of the untreated control at 32 h. In contrast, in the 4th instar larvae of BAW AChE activities increased for 8 h gradually up to 102 % of the activity of the untreated control, and then the activity decreased to 97% of the untreated control at 16 h after treatment.