Imposex is the imposition of male sexual characteristics (notably a penis) on female marine snails and is a sensitive bioindicatorof tributyltin (TBT) exposure. Tributyltin is predominantly used as a biocide in antifouling paints on vessels but has a number of known deleterious impacts on non-target organisms, which have lead to global restrictions on its use. Port Curtis is a large multicargo facility in which TBT has previously been identified as a contaminant of concern. Over 1000 whelks were collected from ten selected sites in Port Curtis and examined for imposex. The prevalence was related to shipping intensity with a decreasing gradient of the number of affected snails from inner to outer harbour. The incidence reported here (0 - 43%) is low in comparison to other Australian studies where up to 100% of snails were affected. The severity of imposex was also not severe indicating that the degree of imposex in this population was in the early stages in comparison to other Australian studies. Organism abundance should not be affected due to recruitment of juveniles from unaffected areas. The imposex survey demonstrated a relationship between exposure to a contaminant and the production of a sub lethal biological response.