High-resolution measurements of the skeletal 13C/12C ratios of massive Porites corals from the Great Barrier Reef and eastern Indian Ocean reveal 13C enrichments culminating sharply at the time of the annual coral spawning event. These 13C skeletal enrichments appear to be driven by the rapid sequestering of 12C required to build the reproductive tissue prior to spawning. Coral spawning is commonly synchronized with the lunar phase and, in this case, is predictable to within 3 days. The results suggest that reproductive modulation of 13C/12C can be exploited as an accurate, built-in time marker for constructing realistic time series with proxy climate data extracted from corals. Recognition of the 13C/12C response to reproduction also facilitates the separation of physiological and environmental influences and should expand the utility of stable carbon isotopes in paleoceanography.