ABSTRACT: Imposex development in female whelks (Prosobranchia: Neogastropoda) has been suggested as a bioindicator of tributyltin (TBT) pollution in the marine environment. The occurrence of imposex in the common whelk Buccinum undatum and the redwhelk Neptunea antiqua in sublittoral areas of Danish waters was related to organotin levels. In contrast, imposex was absent in reference material of N. antiqua collected in the area before 1938. The concentrations of TBT in whole bodytissue of B. undatum ranged from <0.2 to 3.1 ngSn g^-1 and total butyltin (…BT = TBT + DBT + MBT) ranged from 1.2 to 32.9 ngSn g^-1 wet weight.The intensity (i.e. frequency and severity) of imposex was generallyhigher in N. antiqua than in B. undatum. At 14 stations in the Kattegat and Skagerrak, imposex was present in 9 to 89% of B. undatum and 67 to 100% of N. antiqua. N. antiqua seems to be more sensitive to TBT and wouldthus be the better bioindicator.