High-precision Sr/Ca ratios are reported for Porites corals from Davies Reef, at ~19 ¿S on the midshelf of the Great Barrier Reef, Australia. Skeletal growth parameters were also determined from coral slabs using X ray photographs and &ggr;-densitometry measurements. A calibration of the Sr/Ca thermometer has been obtained, using in situ measured sea surface temperatures, which gives Sr/Ca¿103=10.48(¿0.01)-0.0615(¿0.0004)¿T, where T is in ¿C. This calibration applies to different species of Porites and a large range of annual extension and calcification rates, provided sampling follows a major growth axis. Biases of 1¿--2 ¿C have been observed in the low-density margins where growth is hampered by interfering fans of corallites. The analytical and the long-term reproducibility of Sr/Ca-derived temperatures are both better than ¿0.3 ¿C (2&sgr;). Coral growth parameters such as the extension rate or density generally appear to respond to environmental changes such as rainfall but do not affect the Sr/Ca partitioning between coralline aragonite and seawater. No correlation has been found between calcification rate and Sr/Ca variations, consistent with Sr/Ca being mainly controlled by temperature. In the Great Barrier Reef, the interannual variability in SST is primarily related to the El Ni¿o-Southern Oscillation phenomenon and is typically marked by a cold anomaly during the winter preceding the mature warm phase. Instrumental records show that the surface waters of the Coral Sea around 19 ¿S have experienced a significant cooling associated with the El Ni¿os of 1965, 1972, and 1982--1983 which the Porites corals record faithfully. The coral-derived temperatures for the period from 1965 to 1993 show an increase of 1.3 ¿C, which has occurred mainly since 1979. This warming is larger in magnitude but consistent with the trend for the southern hemisphere land and marine temperature records, which shows an increase of around 0.4 ¿C.¿ 1997 American Geophysical Union