Mechanisms of resistance to pirimicarb were surveyed in Iranian populations of the peach-potato aphid, Myzus persicae. Insecticide bioassays were carried out using topical application on three different populations collected from Karaj and Rasht in the Northern provinces. Results of the bioassays indicated that Rasht and Karaj-S populations were susceptible to pirimicarb. A population collected from the college greenhouse (Karaj-R) was resistant to pirimicarb. LD 50 (Ͼ100 ng/insect) of pirimicarb was not calculated for the Karaj-R population due to limited solubility of this insecticide. The activity and PAGE patterns of esterase in resistant (Karaj-R) and susceptible populations showed that one mechanism of resistance to pirimicarb was esterase-based resistance. Esterase activity of the Karaj-R population was 3.25-fold higher than the susceptible population (Karaj-S), and one of the two esterase bands with higher activity in the Karaj-R population showed strong susceptibility to pirimicarb. The acetylcholinesterase (AChE) of the Karaj-R population had lower affinity to artificial substrates, acetylthiocholine, propionylthiocholine and butyrylthiocholine, than Rasht and Karaj-S populations. The I 50 of pirimicarb for Karaj-R, Karaj-S and Rasht populations was 1.89ϫ 10 Ϫ5 M, 1.37ϫ10 Ϫ8 M and 1.37ϫ10 Ϫ8 M, respectively. These results suggest that AChE of the Karaj-R population is insensitive to pirimicarb. Ratios of AChE insensitivity of the Karaj-R population to the Karaj-S population were 1,384.6, 11.8 and 2.7 for pirimicarb, monocrotophos and aldicarb, respectively; however, the ratios were 0.32, 0.15 and 0.10 for carbofuran, propoxure and MTMC, respectively, showing negatively correlated sensitivity to pirimicarbinsensitive AChE.