The Capricorn Reef Complex at the southern end of the Great Barrier Reef lies near the southern limit of the tropics and appears to be in a region that is climatically just adequate for reef growth. The reefs are found on the marginal shelves of the region and grow in a zone 25 or more miles wide. The outer reefs of the Capricorn Reef Complex are located approximately 5 miles shoreward from the shelf edge. Physiographically the complex consists of a gently undulating marine plain 15-30 fathoms deep from which individual reefal mounds rise. The upper surfaces of the reefal mounds (Reef Tops and Reefal Shoals) which are found in the shallow water, high-energy zone form slightly more than 5 percent of the study area. In contrast. the moderate- to low-energy inter-reef zones (Inter-Reef Plains, Inter-Reef Channels, Outer Shelf, and Continental Slope) constitute about 85 percent of the region. Between these extremes is an intermediate zone (Fore-Reef Slopes) which surrounds each reefal unit and grades from the shallow water high-energy zone to the deeper water, low-energy zone. The individual reefs of the Capricorn Group lie in distinct lines. It is suggested that these reefs grew on pre-reef ridges which could be sedimentary structures such as tidal-current ridges, river levees, offshore bars, or barrier islands formed during the Pleistocene low stand of sea level. The elongate shape of the individual reefs is thought to he a result of tidal currents. In detail the distinct shape of each reef is probably the result of wind, waves, biological activity, storms, and reef to reef encroachment. Skeletal material from five organisms form the bulk of the sediments in the complex, but the distribution and ecology of these organisms is poorly known. In general, coral is found growing most abundantly from low tide level to approximately 50 feet in depth on the Fore-Reef Slopes. Encrusting coralline algae creates a smooth pavement on many of the Algal Rims and also encrusts large detritus on the Reef Flats and Fore-Reef Slopes. Halimeda plants live throughout the area, but appear to be a little more abundant in protected places on the reefs and Reefal Shoals. Forams live in the shelter of coral and shingle banks on the Reef Tops, Reefal Shoals, and on the Inner Shallow Shelf to the west of the reef complex. Molluscs are found most abundantly on the sandy part of the Reef Flat, under niggerheads on the Algal Rims, and throughout the inter-reef areas.