In Okinawa, Japan, we found several species of the coral genus Acropora that spawn 1.53h earlier than other mass-spawning acroporids. In six early-spawning species, gamete bundles appear on the surface of the colony (setting) about 1h before sunset, whereas setting occurs about 1h after sunset in the more common late mass spawners. These early spawners are divided further into two sub-groups: three species that release their gametes within 1h after setting and three that release their gametes 1.52h after setting. DNA phylogenetic analyses show that each of these groups forms an independent genetic clade. These genetic relationships suggest that different spawning times form the basis of reproductive isolation among these broadcast-spawning corals. Late spawning appears to be the ancestral state, and the shift of spawning to earlier times may have played a role in the speciation process in these sympatric species.