A fossil colony of the shallow-water, hermatypic scleractinian coral Galaxea clavus (Dana) was collected by research submersible in situ from a prominent terrace 175 m below present sea level along the margin of the Australian continental shelf. Radiometric ages obtained from the original skeletal aragonite indicate that sometime between 13,000 and 17,000 years ago, sea level was glacioeustatically lowered to at least -175 m. Oxygen isotope ratios of carbonate from the fossil Galaxea reflect both seawater temperature and the isotopic composition of seawater at the time the coral was alive. Limits can be placed on these parameters, however, by considering the minimum temperature for the distribution of modern Galaxea, and the maximum increase in seawater ¹⁸O resulting from the transfer of water from ocean to continental ice sheets during glaciation. Although the ¹⁸O values of fossil carbonates by themselves cannot be interpreted in terms of paleotemperature, the combined use of geochemical, biological, and oceanographic data severely restricts the possible range for values of paleotemperature and ¹⁸O of seawater. In-situ sampling of reef corals drowned by the rapid rise of sea level associated with the latest period of deglaciation can yield valuable paleoenvironmental information. In particular, it should be possible to partition the ¹⁸O/¹⁶O ratio of such samples into two components, one representing deposition temperature, and the other reflecting the oxygen isotopic composition of ambient seawater.