In Britain, three species of eel-grass, Zostera marina, Z. angustifolia and Z. noltii, form distinctive stands in the sub-littoral and eu-littoral zones of sand and mud flats. Very few samples of this vegetation were taken and the following account relies heavily on published and unpublished material relating in particular to The Solent (C. R. & J. M. Tubbs), the Thames estuary and Essex (Wyer & Waters 1975; Charman 1975, 1977b, 1979), north Norfolk (Ranwell & Downing 1959, Charman & Macey 1978), Lindisfarne (D. O'Connor), the Moray Firth (Rae 1979), and the west coast of Scotland (A. Currie). There are two difficulties in making use of existing information. First, Z. angustifolia is not consistently distinguished from narrow-leaved forms of Z. marina: this partly reflects the long-standing discussion on the taxonomic status of plants variously described as Z. marina var. angustifolia, Z. hornemanniana or Z. angustifolia. Second, eu-littoral stands have often been described simply as ‘Zostera’ irrespective of whether they comprise Z. angustifolia, Z. noltii or both these species. This has been particularly true of accounts of the grazing of Zostera spp. by wildfowl and a separate note on this important aspect of the conservation value of the vegetation has therefore been appended.