We present a 6-year-long (1978-1984) δ18O and δ13C record from a Great Barrier Reef (Pandora Reef) Porites lutea coral based on near-weekly sample intervals. A sampling technique was designed to minimise any smoothing or distortion of the isotopic record due to complex coral growth, calyx architecture, and calcification at depth within the tissue layer. The arrival-time of the mid-winter minimum sea-surface temperature is very consistent (±2 weeks) near Pandora Reef and provides an annual time-marker offering more precision than the traditional density band chronometer. The improved chronology and high-resolution record demonstrate that signal distortion in Porites, due to calcification within the tissue layer and variable intra-annual coral extension, is generally negligible. Also confirmed is that the arrival-time of monsoonal floods is precisely preserved (±1 week) within the skeleton of Porites. The sensitivity of weekly sampling allows the detection of a subtle warm-to-cool sea-surface temperature anomaly which preceded, by more than one year, a similar temperature anomaly associated with the 1982-83 El Niño in the east Pacific. Sharply higher δ13C values coincide with the time of the annual coral mass-spawning event in the Great Barrier Reef. Recognition of this mass-spawning signal should simplify the interpretation of coral δ13C records and provides an additional, precise time-marker with which to adjust chronologies when intra-annual coral extension is not constant. © 1994.