The ram's-horn snail Anisus vorticulus is one of the rarer and more geographucally restricted freshwater mollusc species in Britain. In this region, A. vorticulus is largely confined to 'traditional' grazing marshes, where it lives in ditches with clean, often calcareous, water with a rich aquatic flora. It has always been a very local species in Britain (Fig. 1), being confined chiefly to Suffolk and Norfolk (mainly the Broads area), the Pevensey and Lewes Levels in East Sussex and Amberley Wildbrooks in West Sussex, with one isolated population at a pond near Staines, Middlesex.