The genus Oryzias containing freshwater (FW) and seawater (SW) species is a potential model for studying mechanisms of osmotic adaptation. In this study, we compared SW adaptability of four Oryzias species, O. javanicus, O. dancena, O. latipes and O. marmoratus inhabiting different osmotic environments. SW adaptability was evaluated at several stages of the lifecycle: (i) survival rates of adult fish after transfer from FW to 50%SW or SW, (ii) spawning ability in FW and SW, (iii) fertilization rates in FW and SW, and (iv) hatching rates in FW, 50%SW and SW. Results obtained agreed with the natural habitat of each species: O. javanicus, which inhabits SW or brackish water (BW), is fully adaptable to both SW and FW at all the stages examined. The BW species O. dancena also revealed high SW adaptability except for the hatching rate. O. marmoratus, confined in FW, exhibited low SW adaptability at all stages examined while O. latipes, another FW species, was adaptable to SW at most stages examined. Based on these results, the role of SW adaptability to the distribution area of each species is discussed.