ABSTRACT: Tributyltin (TBT) pollution has been assessed in NW Brittany since 1992 using the bioindicator imposex in Nucella lapillus. The Bay of Brest is highly polluted by TBT and the outgoing water mass has contaminated the adjacent NW coasts.However environmental conditions have been improving in NW Brittany. The decrease in TBT pollution is most likely due to the reformulated antifouling paints (with slower leaching rates or TBT-free) that are used for civil and military careening activitiesin the Bay of Brest. An increase in imposex levels in 1998 could have been due to the launching of the aircraft carrier ŒCharles de Gaulle¹. It remains unknown when imposex levels reached their maximum because of the lack of annual surveys in 1996, 1997and 1999. This underscores the need for annual surveys when using imposex as a monitoring tool. At 3 locations within the Bay of Brest, new populations of N. lapillus have appeared since 1998. At other stations, populations have grown in number ofindividuals and colonised adjacent rocks. The percentage of less severely affected imposex females (vas deferens sequence stage 3) has increased since 1998. As TBT could soon be banned from antifouling paints, further recovery of favourable environmentalconditions for N. lapillus is expected.