Eggs of the marine medaka, Oryzias dancena were collected and fertilized to observe the temperature-salinity-related cleavage rates and mitotic intervals (0). We investigated the relationship between 0 and five different water temperatures (18, 22, 26, 30, and 34°C) and four different salinities (0, 10, 20, and 30 ppt NaCl). As the water temperature increased, the slope of the first cleavage frequency increased with elapsed time after fertilization, and approximately 30% of fertilized eggs reached the first cleavage frequency at every 15-minute intervals. However, the slope of the first cleavage frequency did not differ significantly between 0 ppt and the other salinities (10, 20, or 30 ppt). At higher water temperatures, the eggs developed more rapidly, but no other developmental process was affected. At higher salinity, the hatching rates of the eggs decreased, and the hatching times were delayed. There was a strong negative correlation between 0 and water temperature as shown in this equation (Y = –1.137X + 75.47, R2 = 0.977, where Y is 0 and X is water temperature). At a constant water temperature, 0 did not differ significantly in 0, 10, 20, and 30 ppt NaCl.