The mesogastropod prosobranches Trivia arctica and T. monacha collected along the coast of Brittany and Normandy between 1988 and 1992 exhibit imposex or pseudohermaphroditism (occurrence of male parts in addition to the female genital system) in response to tributyltin (TBT) pollution. Beside some normal females (stage 0) different imposex stages according to the classification of FIORONI et al. (1991) (3a, 3b and 4 for T. arctica and 3b and 4 for T. monacha) (Fig. 1) can be distinguished and are documented with scanning electron micrographs for the first time. Additional alterations of the genital tract e.g. excrescences on the vas deferens, a coiled oviduct, a bi- or trifurcated penis or several penes (2-5) are described. Neither TBT-induced sterilization nor sex change occur. TBT accumulation in soft parts shows sex-related differences. The VDS (vas deferens sequence) index, uncubed RPS (relative penis size) index and average females penis length of a population are dependent from TBT concentrations in ambient sea water and TBT body burden. A statistical study, based on an analysis of natural populations of T. arctica, T. monacha and Nucella lapillus allows a comparison of the specific TBT sensitivity of the three TBT bioindicators. N. lapillus exhibits a lower threshold for imposex development and a higher TBT sensitivity, but both Trivia species proved to be suitable TBT bioindicators.