In 1987 the UK Government banned the use of tributyl tin (TBT)-based antifouling paints on small boats of less than 25 m. Following initial control measures taken in 1986, a program of research was started to monitor concentrations of TBT residues in estuarine waters and sediments and to observe faunistic changes in highly contaminated estuaries. As part of this program, the size-frequency and abundance of Littorina littorea populations have been recorded in the estuaries of the rivers Crouch (Essex) and Hamble (Hampshire). Since the TBT ban, the concentration of TBT residues in water and sediments has been steadily declining. In both L. littorea populations, the frequency of O-group individuals has increased markedly, and there has been a simultaneous decrease in TBT residues in L. littorea tissues. Furthermore, plankton surveys of the River Crouch show that the numbers of L. littorea eggs and veliger larvae have progressively increased, suggesting that TBT may have impaired periwinkle reproduction and/or survival of the eggs and larvae. Subsequent laboratory experiments have indeed shown that reduced egg production was the probable mechanism of action, but the imposex associated with exposure to TBT in dogwhelks (Nucella lapillus) has not been seen.