Neogastropod snails are gonochorist, i.e. the sexes are separate, but when exposed to tributyltin, a biocide used in marine antifouling paints, the female becomes masculinized, typically developing a penis and a vas deferens. This process is termed ‘imposex’ (describing a superimposition of male sex organs onto the female) and the phenomenon has been recorded in many species world‐wide. Evidence suggests it is caused by an hormonal imbalance induced by tributyltin at a very low ambient concentration of just a few nanograms per litre. Thus, imposex can be used as a highly sensitive bioindicator of tributyltin pollution, especially as the degree of its development appears to be dose‐dependent. In the Mediterranean, onlyHexaplex trunculus(Muricidae) has been studied in this context, but information is limited and the status ofH. trunculuspopulations along the Italian coast remain to be investigated in detail. The present survey is based on examinations of nearly 3000 specimens collected at various localities around Italy. The development of imposex inH. trunculusis described and its ecological implications for the species are discussed.