Many marine organisms have a specific reproductive timing, but its adaptive nature has not been directly tested because of the difficulty of manipulating reproductive timing in the field. Egg survival in relation to the timing of egg release in the puffer Takifugu niphobles, an intertidal spawner, was assessed by experimentally transplanting eggs to several tidal zones, and the observed pattern of variation in spawning time across environments (a pebble vs. a sandy beach) was examined to see if it was consistent with the presumed selection regime. The vertical distribution of eggs and the observation of spawning zones showed that substrate characteristics affected the fate of released eggs: at the pebble beach eggs were stranded in the high intertidal zone, while they were washed out toward the lower zone at the sandy beach. The egg-transplant experiment showed that egg survival was lower in the high zone than in the middle and low zones at both the pebble and sandy beaches, indicating that individual fishes should spawn earlier relative to the tidal cycle. However, the selection pressure for thefish to spawn earlier was more intense at the pebble beach than at the sandy beach, which was attributed to the difference between the beaches in the degree to which eggs were transported seaward. At the pebble beach (1) the timing of spawning was more strictly synchronized with the tidal cycle, (2) a day's spawning started, peaked, and ended earlier relative to the tidal cycle, and (3) a day's spawning was concentrated in a shorter time, than at the sandy beach. These temporal patterns of spawning are allexpected from the presumed selection regime across the beaches, suggesting that there is an evolutionary response of the trait to this selective force. This study explains interpopulation variation in reproductive timing of a marine fish by combining experimental manipulations of the reproductive timing with classic comparative methods. This experimental approach performed in different ecological settings is essential to further understanding of the ecology and evolution of temporal patterns of reproduction.