Culex pipiens (L.) (Diptera: Culicidae) is the most important medical insect in many parts of the world. Biological and natural chemicals have many advantages over the traditional ones in case of mosquito control. The efficacy of two insecticides belonging to different groups chlorpyrifos-ethyle (oranophosphate) and Bacillus thuranginsis var israelinsis) (bio-insecticides)were evaluated against the field and laboratory individuals of late 3rd instar larvae of Culex pipiens at different concentrations and three periods of exposure under laboratory conditions. Results obtained showed that the laboratory strain showed higher susceptibility to the tested insecticides than the mosquito populations collected from Abo-Rewash City, Giza Governorate and the mortality percentage was increased gradually with increasing the insecticide concentrations and the mortality percentage showed significant differences between concentrations and control. Also, The activity of acetyl cholinesterase (AChE), the glutathione S- transferase (GST), Total proteins and the activities of both Aspartate aminotransferase AST(GOT) and Alanine aminotransferase ALT(GPT) were determined were determined after 72 hrs of insecticidal exposure.