Abstract: The synergistic mechanism of a mixture of triazophos and fipronil to the 4th-instar larvae of Mythimna separate was studied by the analysis of interactions between triazophos and fipronil on cuticular penetration,biotransformation and target site.The results suggested that triazophos and fipronil promoted each other to penetrate the cuticle of Mythimna separate in the mixture.The interaction between triazophos which had competitive inhibition on acid phosphatase(ACPase)and fipronil which had mixture inhibition (non-competitive and counter-competitive inhibition) on ACPase caused the remarkable increase in the inhibition ability of the mixture on ACPase.Joint-inhibition coefficient of the mixture to glutathione S-transferase(GSTs) was 425.87,which indicated that the mixture was significantly synergistic to GSTs.The interaction between triazophos and fipronil on AchE was weak.The increase of cuticular penetration rate and the enhancement of inhibition to the activity of ACPase and GSTs may be the major synergistic mechanisms of the mixture of triazophos and fipronil to Mythimna separate.