This survey was carried out to describe the sublittoral fringe communities on and around the Mid-Wales Sarns (Sarn Badrig, Sarn-y-Bwch and Cynfelin Patches), and to assess their scientific interest and nature conservation importance. The Sarns (reefs) are interesting geological features, being shallow sublittoral glacial moraines. The survey was conducted from the hard boat St Margaret', and based at Abersoch on the Lleyn Peninsula in North Wales. Daily trips were made out to the reefs, and diving operations were based on the hard boat with inflatable tender for divers. At each site surveyed, records were made of abundance of plants and animals, and habitats were described. Suction samples were taken of sediment infauna at several sites. Photographs were taken to illustrate habitats, communities and species. The Sarns were remarkable for the extensive areas of distinctive communities onunstable' substrata. A wide variety of algae, particularly Phaeophyta (brown algae), was recorded. At most survey sites, algae were dense and animal species were fewer and less varied. Twenty sites were surveyed, and descriptions of the different habitats and communities found on the reefs are given. The reefs were composed entirely of `unstable' substrata, of a wide range of sizes from large boulders down to fine sand. Distinctive communities dominated by ephemeral algae, were present in the sublittoral fringe on the reef crests, where there were mixed substrata exposed to fast tidal currents and strong wave action. In slightly deeper water, dense foliose algae dominated all hard substrata, with few animals present. Between 4 m and 9 m, Halidrys siliquosa forests covered extensive areas on boulders and cobbles in sediment. Pebbles were patchy but with a characteristic flora. In deeper water off the ends of the reefs, animal communities with a wide variety of species were present on tideswept boulders. Areas between the reefs were mainly covered with sand, and not investigated thoroughly. The conservation importance of the reefs as a whole is assessed, and of the 10 community types described. 11 algal and 4 animal species are identified as being of particular scientific interest. Many of the communities described are considered of regional or local importance; however the current-swept reef crests and the extensive Halidrys forests are considered to be of higher importance. The reefs as a whole are probably unique sublittoral features in the British Isles. Further work is recommended on Sarn-y-Bwch and Cynfelin Patches, both inadequately surveyed. The large boulder habitat, and areas at both the seaward and landward ends of the Sarns deserve further study.