Seven field strains of Culex pipiens pallens were evaluated in the summer of 2004 for their susceptibility to three insecticides. Larval bioassays were carried out in accordance with WHO mosquito susceptibility tests. Field samples showed resistance to dichlorvos and propoxur. Low levels of resistance to phoxim were detected compared with the laboratory-susceptible strain. Quantitative analysis showed that co-amplification of estα2 and estβ2 occurred in the seven field Cx. pipiens pallens. However, the levels of transcription of estα2/estβ2 genes in the field strains indicated a 15.9:1 ratio of estβ2 cDNA over estα2 cDNA. Such observed differential transcription of the estα2/estβ2 genes showed that the promoters regulating their expression are independent and of different strengths. Regression analysis of expression level of estα2/estβ2 gene and LC₅₀ value to insecticides showed a significant correlation between expression level of estα2 gene and LC₅₀ value to dichlorvos. However, correlation of estα2 gene and LC50 value to propoxur and phoxim, or estβ2 gene and three kinds of insecticides, was not significant.