The resistance of mosquitos to conventional insecticides created an urgent need for using alternative insecticides. In this study, the toxicity of the neonicotinoids (acetamiprid and thiamethoxam) was evaluated against the 3^rd  larval instar of Culex pipens. In the larvicidal assay, acetamiprid and thiamethoxam showed promising larvicidal activity against Cx. pipiens, with LC₅₀ values of 0.0093 and 0.0305 ppm after 24 hrs, 0.0078 and 0.0206 ppm after 48 hrs and 0.0065 and 0.0137 ppm after 72 hrs of insecticidal exposure. The activity of acetylcholinesterase (AChE), carboxylesterase, (α-esterase), (β-esterase), and glutathione S- transferase (GST) were determined after 72 hrs of insecticidal exposure. Acetamiprid showed a significant increase in the activity level of AChE, GST, carboxylesterase, α, and β- esterases than thiamethoxam. The hemocytes of the treated 3^rd larval instar of Cx. pipens were sensitive to both insecticides and showed remarkable deformation compared with control. Therefore, both detoxification enzymes and physiological resistance play a great role in neonicotinoids detoxification.