As part of the ’Atlasproject Nederlandse Mollusken’, the recent distribution of many Dutch molluscs is under investigation. Mercuria confusa (Von Frauenfeld, 1863), reported from the Netherlands for the first time in 1958 and extensively investigated in the sixties, is one of the species of special interest. As a result of the completion of the ’Delta-works’, many aquatic habitats in the southern part of the Netherlands have drastically changed, including many ’Mercuria-localities’. Although rather rare, Mercuria still is a member of the Dutch malacofauna. However, most of the now re-discovered populations are obviously remnants of a previously much larger distribution. The recent distribution shows a highly ’patchy’ pattern, an indication of a population in decline.