The red imported fire ant( Solenopsis invicta) is a global major invasive pest, and has caused significant economic, social and environmental impacts since its invasion to mainland of China in 2004. To date, chemical control has been the most effective measure. However, the long-term use of chemicals would lead to an unexpected rebound. To understand the risks and explore the mechanisms of detoxification or induction to insecticides in S. invicta, the O-demethylase activity and expression of cytochrome P450 genes of workers and queens, and the effects of chlorpyrifos and fipronil exposure in workers were investigated. Biochemical assays showed the O-demethylase activity of cytochrome P450 was significantly higher in workers than in queens(1.66-fold), and was significantly induced in workers exposed to chlorpyrifos and fipronil, reaching a maximum(3.00- and 1.95-fold) at 48 h and then decreasing dramatically compared to controls(exposed to acetone counterpart). The relative expression levels of 12 cytochrome P450 expressed sequence tags(ESTs) in workers were significantly higher than in queens(from 2.3- to 36.4-fold). Multiple cytochrome P450 genes(except 9E4) were co-up-regulated(from 1.5- to 2.86-fold) in workers exposed to fipronil. These results indicated that the increased O-demethylase activity may result from the increased transcription levels of cytochrome P450 related to detoxification of insecticides in S. invicta. It appears that cytochrome P450 plays an important role in enhanced metabolic detoxification of insecticides. At the same time, it also provides the theoretical basis for resistance management and rational usage of in secticides to control S. invicta. The red imported fire ant (Solenopsis invicta) is a major major invasive pest, and has caused significant economic, social and environmental impacts since its invasion to mainland of China in 2004. However, the long-term use of chemicals would lead to an unexpected rebound. To understand the risks and explore the mechanisms of detoxification or induction to insecticides in S. invicta, the O-demethylase activity and expression of cytochrome P450 genes of workers and queens, and the effects of chlorpyrifos and fipronil exposure in workers were investigated. Biochemical markers showed the O-demethylase activity of cytochrome P450 was significantly higher in workers than in queens (1.66-fold), and was significantly induced in workers exposed to chlorpyrifos and fipronil, reaching a maximum (3.00- and 1.95-fold) at 48 h and then presumption to control (exposed to acetone counterpart). The relative expression l Multiple cytochrome P450 genes (except 9E4) were co-up-regulated (from 1.5- to 2.86-fold) fold) in workers exposed to fipronil. These results indicated that the increased O-demethylase activity may result from the increased transcription levels of cytochrome P450 related to detoxification of insecticides in S. invicta. It appears that cytochrome P450 plays an important role in enhanced metabolic At the same time, it also provides the theoretical basis for resistance management and rational usage of in secticides to control S. invicta.