Tributyl tin (TBT) is a major component of anti-fouling coating used to prevent attachment of aquatic organisms to boats and ships. TBT causes imposex, a condition whereby female marine gastropods develop male sexual characteristics and become sterile. In the present study, it was determined that TBT is a non-competitive inhibitor of glutathione-S-transferase (GST), an enzyme responsible for elimination of steroids in organisms. The efficiency of molluscan GST from the garden snail, Helix aspersa, is greatly reduced by the presence of 10 µM and 100 µM TBT. This may explain the build up of androgens in aquatic molluscs.