We have obtained a Helicoverpa armigera strain (MonoR) showing high level resistance to monocrotophos by 15 generations of selection on a weakly resistant field-caught strain. Bioassays using various synergists show a major role for esterases in the resistance, and little if any role for cytochrome P450s or glutathione S-transferases. Biochemical data also show elevated levels of esterase activity in the resistant line. There were also minor differences among our strains in the kinetics of acetylcholinesterase activity and in its susceptibility to monocrotophos inhibition, and sequence comparisons revealed one amino acid substitution in AChE1 but none in AChE2 in the MonoR strain. However the AChE1 substitution does not match any others linked to resistance in this or other species and would be located well away from the enzyme active site, so it may not be causally involved with resistance. Native PAGE shows several individual esterase isozymes are more intensely staining in the resistant line and native Western analysis with an antibody against Clade 1 esterases show that several of these belong to Clade 1 and that their greater staining intensity in MonoR is due to greater amounts of the respective enzymes. Proteomic analysis of gel slices also matches the overexpressed bands with up to six Clade 1 enzymes, with some involvement also from esterases in three other Clades. Two of the isozymes that are overexpressed in MonoR are also overexpressed in a fenvalerate resistant line and proteomic analysis of that line bears out previous proteomics of another fenvalerate resistant line which also implicates Clade 1 isozymes.