Marine sediments and gastropod, Thais clavigera, were collected from industrialized areas and major harbors in Korea, to assess contamination sources and ecological risk to marine organisms by butyltin compounds such as tributyltin (TBT). TBT concentrations in sediments ranged from <LOD to 5372 ng Sn/g dry weight, and were comparable to or higher than concentrations in big harbors of other countries. The highest TBT concentrations were found in sediments close to shipyards in surveyed bays, suggesting that shipyard activities are a major contamination source of butyltins in Korean coastal waters. About half of the sediments showed potential risk of exceeding the guidelines and values proposed in in vivo studies. The occurrence of imposex in T. clavigera was 100% at all locations, indicating that the present levels of TBT contamination in industrialized bays in Korea are at concentrations reported to induce toxic effects on marine organisms, despite TBT's having been legally banned in Korea.