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