This survey was undertaken as part of a general study of the Menai Strait, commisioned by NCC, required to assist in decisions related to its designation as one of Britain's Marine Nature Reserves. Parallel surveys of the soft substrata and sublittoral hard substrata have also been conducted. The work was carried out during the summer of 1982 and subsequently supplemented by visits to less fully recorded sites. The purpose of this survey is to provide a topographical and biological description of the Anglesey and Arfon sides of the Menai Strait, concentrating on the hard or stable substrata and the animals and plants which live in these habitats. Flora and fauna lists are appended which cover all substrata. They are derived mainly from records in UCNW, Bangor, including those at the Marine Science Laborities, the School of Animal Biology and the School of Plant Biology. Additional records are from published material or personal information. Hard substrata have been studied rather longer than the soft, and littoral recording has been going on for longer than sublittoral work, but it has been highly selective; some areas of the Strait have been the subject of collecting and recording for many years; others have seldom been visited. This implies a judgement on the relative richness and interest and, in general, this difference in relative richness and interest is borne out by the observations of the present survey which may be summarised as follows: 1) The Menai Strait is an area of very unusual shores of great ecological interest. 2) This is because of a wide variety of rock types and other substrata and especially because these are subjected to strong tidal currents which are most rapid where wave action is minimal. This interest is not uniform, however, and some parts of the area stand out as particularly exciting: 1) Church Island and the adjacent shores, 2) Black Rock, 3) Puffin Island and Penmon, 4) Ynys y Moch and adjacent shores, 5) Gorad y Gyt to the Britainnia Bridge, 6) Llanddwyn, 7) Dinas Dinlle. Black Rock and Ynys y Moch are included in this list because of their past glories rather than their present depressed state. These shores make a particularly strong case for a policy of protection to prevent further damage by bait-digging. In the case of Ynys y Moch, some active restoration measures are also practicable but the recovery of shores like Black Rock will require a long period of peace. Llanddwyn and Dinas Dinlle are not high on the list because, though both include species not found elsewhere in the Strait, neither is particular rich by comparison with boulder or rocky shores farther along the north Wales coast. However, all these shores are of great interest and worthy of special efforts to conserve and protect. There are also stretches of the Strait where the littoral interest is not great. These must include the shores from Beaumaris to Ynys Gaint; Plas Newydd to Tal y Foel; Britainnia Bridge to Caenarvon. Further investigation is merited into possible reasons for absence or variations in flora and fauna on different sides of man-made structures in some isolated localities. There is no doubt that the Church Island region and whole of the Swillies shores between the bridges would merit NNR status in any assessment of British coasts. The inclusion of other parts of the Menai Strait must depend partly on the philosophy behind marine reserves. Thus the limits of Puffin Island to Llanddwyn/Dinas Dinlle define a clear geographical area and include, in the last two shores, typical (though not the richest) examples of certain habitats. The NE end of the Strait, however, is intrinsically more interesting than the SW and the Puffin Island, Black Rocks, Cae Coch and Penmon shores are worthy of inclusion on their own merits. This implies three possible limits: 1) The central reach, for which an overwhelming case exists, 2) The central and NE reaches, from Puffin Island to the Brittainnia Bridge, for which a strong case can be made on ecological grounds, 3) The whole of the Strait, for which the case tends to be more a matter of geographical unity. The historical aspect of conservation must also be considered. There are two main issues related to different time scales. In the longer scale, there are a series of structures used since the middle ages in a method of fishing, peculiar to the Menai Strait and surviving in various states of dilapidation. A major gap in our knowledge appears from this survey relating to these man-made structures and the activities of fishermen and other users of the waters in previous years. In the last 10 - 20 years, there has been an irreplaceable loss of local knowledge by the death of so many elderly people who had earned their living from the Straits. It is now probably too late to save more than a fraction of this once widespread knowledge, but an effort should certainly be made before the last remnants disappear. These Goradau should be made the subject of special conservation measures; one or more could be restored and used experimentally to establish the methods and efficiency of the fishery. Fundamental biological information would almost certainly be acquired at the same time. The most obvious Gorad on which to begin would be Gorad Bach near Beaumaris Life Boat House, which was the most recently used trap and where knowledge of the methods still survives. On a shorter time scale, extensive recording of animals and plants was carried out in the years 1887 - 1891 on Puffin Island, in other parts of the region in the next few years. This gives a picture of these shores at a point in time since when changes have occurred. We are approaching a hundred years since that surge of activity and another intensive recording period would produce very interesting comparisons and might be a very appropriate start to the management plan of a new reserve. In this connection it might be added that there are areas of potential interest which have never been adequately explored. These include Ynys Gorad Goch, the Brittainnia Rock, the Platters and Swilly rock; it is ironic that these are within the central area which possesses, in Church Island, the most intensively studied shores of all. In fact, the establishment of a Marine Nature Reserve in the Menai Strait is not merely an attempt to preserve a very well known stretch of coast - it can be a stimulus to more active investigation of a surprisingly incompletely explored area. Records currently considered sensitive have been removed from this dataset.