Abstract:  A laboratory susceptible strain （YSS） of cotton aphid, Aphis gossypii (Glover), was selected with omethoate in successive generations in the laboratory to develop a resistant strain (YRR ). After 24 generations, the resistance ratio of YRR strain was increased by 124.7-fold compared with the omethoate-susceptible strain. Carboxylesterase activity was significantly lower in the omethoate-resistant strain than in the omethoate-susceptible strain when α-naphthyl acetate （α-NA）was used as substrate. A  carboxylesterase gene had been fully cloned and equenced from both omethoate-susceptible and resistant strains. Comparison of both nucleic acid and deduced amino acid sequences revealed  four nucleotide acid differences between the omethoate-susceptible and resistant strain, which resulted in four amino acid substitutions (His¹⁰⁴→Arg, Ala¹²⁸→Val, Thr³³³→Asp, Lys⁴⁸⁴→Arg). Knowledge of the structure of a related enzyme acetylcholinesterase) suggests that one of these substitutions (His¹⁰⁴→Arg) lies within the active site of the enzyme. The GenBank accession numbers of carboxylesterase genes of omethoate-resistant (YRRAA) and susceptible (YSSAA) A. gossypii are AY485216 and AY485214，respectively.

Key words: Aphis gossypii, resistance, carboxylesterase gene, cloning, gene mutation, omethoate