Coral reefs are renown for biodiversity. Among fishes, the Labridae is one of the largest groups on coral reefs, including about 500 species of labrids and about 100 species of scarids. Recent studies on the molecular phylogeny of the Labridae have suggested that sexual selection, rather than modifications in feeding apparatus, was a major factor in their diversification. The present study indicates that spawning time would have influenced the intensity of sexual selection because female labrids and scarids choose their mate by sight. They spawn pelagic eggs usually in the daytime, although most of the pelagic egg spawners of other families usually spawn at dusk responding to low levels of planktivore activity. We found interspecific difference in spawning time among the Labridae (30 species of labrids and 13 species of scarids) at a reef-edge spawning site of Iriomote Island, Okinawa. Most labrids spawned following high tide, only a few species at dusk, and most scarids spawned in the early morning. Species of the most primitive lineage spawned at dusk. It is suggested that transition to daytime spawning would have facilitated sexual selection and diversification in the Labridae.