Abstract: In previous works tolerance (up to the 10th generation) to each of six monoterpenes,linalool, S-carvone, geraniol, camphor, γ-terpinene and fenchone and two phenylpropanoids,estragole and E-anethole, was selected for S. oryzae and C. pusillus in the laboratory. Thisprocess could possibly have involved changes in physiological and/or biochemical characteristicsof the insects since one possible mode of action of monoterpenoids in stored product pests is byinhibition of acetylcholinesterase (AChE). The extraction and activity of the enzyme fromS. oryzae and C. pusillus strains from either susceptible or tolerant (to each monoterpenoid)insect populations was carried out. Activity of the enzyme at different concentrations of substrate(acetylthiocoline iodide) was measured by the Elman’s method. Overall C. pusillus gave moreamount of AChE extracted than S. oryzae. In both insects, the amount of enzyme extracted fromsusceptible populations was greater than in the resistant strains, with the exception ofC. pusillus population tolerant to camphor, linalool or geraniol. In addition, the enzymaticactivity in monoterpenoid susceptible insect populations was superior to the resistant strains inthe two pests, although there were no significant differences among the majority of populations.