Imposex, specifically caused by TBT pollution, refers to the superimposition of male sexual characteristics in gastropod females. Seasonal variation of imposex intensity in Thais clavigera from both slightly and severely contaminated sites in Hong Kong waters was studied from 1988 to 1999. The male penis length showed significant difference between both sites and seasons. It was shortest during late autumn and early winter (October to December) and longest during spring and early summer (February to June). Female penis length also showed significant difference between sites. It did not change seasonally, however. The RPS (Relative Penis Size) index was the highest during autumn and early winter, and the lowest during spring and early summer. The VDS (Vas Deferens Sequence) index remained stable throughout the sampling period. This study showed that VDS index is a better indicator when we compare relative intensity of imposex. The comparison can only be meaningful provided the samples from different locations are taken during the same season. Imposex, specifically caused by TBT pollution, referring to the superimposition of male sexual characteristics in gastropod females. Seasonal variation of imposex intensity in Thais clavigera from both slightly and severely contaminated sites in Hong Kong waters was studied from 1988 to 1999. The male penis length showed significant difference between both sites and seasons. It was shortest during late autumn and early winter (October to December) and longest during spring and early summer (February to June). It did not change The RPS (Relative Penis Size) index was the highest during autumn and early winter, and the highest during spring and early summer. The VDS (Vas Deferens Sequence) index remained stable throughout the sampling period. This study showed that VDS index is a better indicator when we compare relative intensity of imposex. The comparison can only be meaningful provided the samples f rom different locations are taken during the same season.