Culex quinquefasciatus Say, a widely distributed mosquito in tropical and subtropical areas, is the most important vector of the filarial parasite Wuchereria bancrofti, the causative agent of lymphatic filariasis. In India, filariasis is endemic in 17 States and six Union Territories, putting about 553 million people at risk of infection. Vector control, which aims to prevent pathogen transmission through interventions targeting adult mosquito vectors, is a significant component of control of the disease. Chemical-based control represents a chief strategy in the management of mosquito vectors; however, continuous application of insecticides has led to the development of resistance in many mosquito vectors around the world. The current study aims to observe the variability of expression of carboxylesterase isozymes that play a role in detoxifying insecticides into non-toxic compounds, in different life stages of Culex mosquitoes, to understand levels of insecticide susceptibility that may be used in integrated mosquito management for efficient vector control. C. quinquefasciatus were collected from different localities of sub-Himalayan West Bengal, India, and adult and larval bioassays were performed against one organophosphate insecticide (chlorpyrifos) and two synthetic pyrethroid insecticides (deltamethrin and lambda-cyhalothrin). The activity of α- and β-carboxylesterases (quantitative assay) were measured in larvae and adults of C. quinquefasciatus using a microplate assay, and measured qualitative expression by native polyacrylamide gel electrophoresis. The study shows a differential activity of α- and β-carboxylesterases both in quantitative and qualitative assays. The quantitative assay reveals that larvae exhibit a 12.2-fold higher level activity of α-carboxylesterase and about 5.0-fold higher level of activity of β-carboxylesterase than adults. Some carboxylesterase isozymes, i.e., α-Est I-IV, α-Est VII and α-Est XI-XV were exclusively expressed in larvae, whereas α-Est V-VI and α-Est IX were expressed only in adults. In larvae, all β-Est I-IX were expressed, while in adults only β-Est IV-V was expressed. The results of adult and larval insecticide bioassay are also as per the above findings showing an LC90 value of 0.017 ppm, 0.097 ppm and 0.072 ppm in the larva, and LC90 value of 0.0015 ppm, 0.721 ppm, 0.364 ppm in adults against chlorpyrifos, deltamethrin and lambda-cyhalothrin, respectively.