cDNAs encoding two acetylcholinesterases (AChEs) were isolated from the peach potato aphid, Myzus persicae. MpAChE1 was orthologous and MpAChE2 was paralogous with the ace of Drosophila melanogaster. The deduced amino acid sequence of MpAChE1 cDNA was identical between the pirimicarb susceptible and resistant strains. However, a single amino acid substitution of Ser43lPhe on MpAchE2 was found in the pirimicarb resistant strains. This substitution was located in the acyl pocket of the enzyme and was thought to alter the ligand specificity. (C) 2003 Elsevier Science (USA). All rights reserved.