Measures of imposex in the DogwhelkN. lapillusand the Common WhelkB. undatumwere used, together with analyses of butyltins in tissue and sediment samples, to assess current levels of TBT contamination in four sealochs, which are subject to different kinds and intensities of boat use and fish farming. Imposex inN. lapillusis a specific response to TBT and some related organotins and, as would be expected therefore, indices of imposex correlated with concentrations of butyltins in tissues. Data from this species, and sediments, indicate that TBT contamination in three of the sealochs (Loch Sween, Loch Crinan and Killary Harbour) is now low. It has decreased significantly in Loch Sween and Loch Crinan during the past decade. There were higher levels of pollution in Sullom Voe, where oil tankers are a source of TBT. This was reflected in measures of imposex and concentrations of butyltins in tissue samples but not of those in sediments, which were surprisingly low. Imposex was recorded in most samples ofB. undatumexamined in this study but it is not a useful indicator of butyltin contamination. The condition was severe in some places where pollution was low and, conversely, absent or poorly developed in others, where it was high. There was no correlation between the severity of imposex and tissue concentrations of butyltins in this species.