W. M. Davis considered that ‘none of the elevated reefs of the Solomon Islands is more remarkable than the emerged barrier reef which skirts the north-eastern side of the long island of New Georgia’ (Davis 1928, pp. 397-398), and this reef was one of the main objects of geomorphic work during the Royal Society Expedition to the British Solomon Islands. Davis did not visit the Solomons and based his discussion entirely on charts, and apart from the Admiralty Pilot , the Pacific Islands Handbook , and brief mention in the publications of the Geological Survey of the British Solomon Islands, the New Georgia reefs have remained entirely unstudied. The Marine Party of the Expedition visited New Georgia three times: first, from 2 to 9 August 1965, concentrating on the northern Marovo Lagoon; secondly, from 26 August to 8 September, when the biologists returned to the same area, and geomorphic work was extended south to the southern end of Japuchajomo and north to Lumaliha; and thirdly, from 14 October to 18 November, when M. V. Maroro was engaged in echosounding and bottom sampling with the writer and Dr P. E. Gibbs in Marovo , Kolo, Togavai and Gerasi Lagoons. A brief visit was also paid by the whole marine party to Ulukoro and Batuona Islands, Wickham Anchorage, en route to Gizo, on 3 September. This paper reports on studies of reef geomorphology carried out during these visits, and includes studies made by diving at Matiu Island in collaboration with Dr S. A. Wainwright. The sedimentological and bottom fauna studies carried out during the coring, dredging and grab-sampling programmes on the lagoon floor will be reported separately.