Culex quinquefasciatus Say is an important pest species and a vector of multiple pathogens. Insecticide applications are necessary for the effective control of mosquitoes. In the current study, a laboratory population of Cx. quinquefasciatus was exposed to chlorpyrifos for 15 consecutive generations and then assessed for the changes in detoxification enzyme activities before and after exposure to Metarhizium anisopliae (Metschn.) Sorokin and Beauveria bassiana (Bals.) Vuill. during 14th–15th generations. Activities of acetylcholinesterase (AChE), glutathione S-transferase (GST), esterase (EST), acid phosphatases (ACP), and alkaline phosphatases (ALP) were increased in the chlorpyrifos-selected (Chlor-SEL) population in relation to an unselected (Un-SEL) population. The resistance ratio of Chlor-SEL 15th generation (G₁₅) was increased 3,583-fold against first generation (G₁) and 6,026-fold against the Un-SEL population. The results depicted maximum activities of ACP (83.48), ALP (65.54), GST (13.047), EST (10.42), and AChE (4.86) µmol/min of mg/ml protein at G₁₅ after consecutive chlorpyrifos applications. The Chlor-SEL populations at G₁₄–G₁₅ were treated with different concentrations of M. anisopliae and B. bassiana for possible suppression of enzymatic activities. Activities of ACP were suppressed to 24.22 µmol/min of mg/ml protein at G₁₅ when exposed to B. bassiana and 22.40 µmol/min of mg/ml protein at G₁₄ after exposure to M. anisopliae. The suppression of detoxification enzymes by application of fungi in resistant population of Cx. quinquefasciatus will aid in the mosquito's management programs.