To clarify the relationship between water temperature and the embryonic period of Pacific rainbow smelt Osmerus dentex, we incubated eggs in the laboratory with water-temperature conditions close to those of the spawning season (8-14°C). The relationship between the mean incubation temperature (T℃), and mean embryonic period (D, days) were as follows: D = 147.3/(T-3.4). The hatching rates 8°C and 10°C were significantly higher than those at higher temperatures (12°C and 14°C). From these results, it was possible to accurately estimate the incubation period and appropriate temperature for hatching.