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Abstract Modified acetylcholinesterase (AChE) phenotype (MACE) caused by a mutation in the ACHE gene (ace2) leading to pirimicarb resistance in Myzus persicae (Sulzer) has been reported worldwide. Pirimicarb resistance levels and AChE susceptibility to this insecticide were investigated in 19 field populations collected in six provinces of China, in compared to a susceptible laboratory population, five of the 19 field populations were susceptible to pirimicarb with resistance factors (RF) < 5, whereas the other 14 populations were found to be resistant to pirimicarb (RF ranging from 16.3 to 101.9). The residual activity of AChE after exposure to pirimicarb varied significantly from 10.7% to 91.4% among these populations, three of them exhibited extremely low AChE sensitivity to the pirimicarb. In addition, residual AChE activity was positively correlated with increased resisitance to pirimicarb. Nine populations showing a wide range of RF values were selected to investigate S431F mutation frequency in ace2. The mutation frequencies in these populations were all > 85%. The RF values of the populations were positively correlated with the recorded frequencies of S431F mutation. These results demonstrate that MACE is strongly involved in the resistance to pirimicarb observed in populations of M. persicae in China.