In 1973 and 1974 two surveys were conducted of all reefs falling within four transects in the region of the Great Barrier Reef thought to represent the southern section of the area in which large populations of Crown-of-thorns starfish (Acanthaster planci (L.)) were active. The surveys were carried out by towed or free-swimming snorkel divers who covered the entire perimeter and the majority of back reef and lagoonal areas of reefs visited. Of 25 reefs surveyed ten were found to carry large populations of A. planci during one or both surveys. The interval between surveys was 11 months for transects 1-3 and six months for transect 4. During the interval A. planci populations increased on two reefs but decreased on five although the overall number of A. planci sighted was similar for the two surveys. The surveys revealed considerable changes in the density and distribution of A. planci populations on individual reefs but did not produce evidence of migration of adult A. planci onto previously unpopulated reefs. Dense populations of A. planci were studied by SCUBA divers; the highest population density recorded during the surveys was considerably less than that reported during earlier surveys of starfish populations on the Great Barrier Reef. Although there were areas on a number of reefs in which considerable coral mortality appeared to have been caused by the predation of A. planci they were generally restricted and interspersed with areas with a moderate cover of live corals. © 1976.