Induction imposex of gastropod snails by organotins has been reported worldwide. However, the relationship between imposex and detoxification capability is not well studied in molluscs. In the present study, the relationship between the mixed function oxidase (MFO) system and the degree of imposex in drills Thais spp. was investigated. Thais clavigera, T. rufotincta and T. sp. were collected from the west coast of Taiwan. The degree of imposex was determined by percent imposex, relative penis size (RPS) and vas defenes sequence (VDS). Enzymes of cytochrome P450, NADPH-cytochrome c reductase, EROD¡]7-ethoxyresorufin-O-deethylation¡^ and cytochrome P420 (an inactive form of cytochrome P450) in the MFO system were examined. In the three Thais species, the degrees of imposex expressed as percent imposex, RPS and VDS were 71-100%, 8.8-62.5% and 0.7-2.5, respectively. Contents of cytochrome P450 and cytochrome P420 were 0.043-0.097 nmol/mg protein and 0.129-0.296 nmol/mg protein, respectively. The activity of NADPH-cytochrome c reductase was in the range of 1.52-6.73 nmol/min/mg protein. The activity of EROD was not detected. Negative trends between the degree of imposex and the contents of cytochrome P450 and cytochrome P420 were found in all the Thais species. And, a positive trend betweed the degree of imposex and the activity of NADPH-cytochrome c reductase was also observed. Based on the results, it is suggested that the induction of imposex was possibly due to the imbalance of steroid hormone metabolism caused by the inhibition of cytochrome P450 activity.