Abstract: Rachycentron canadum is a thriving mariculture species for offshore cage in southern Mainland and Taiwan of China, due to its rapid growth rate and high quality flesh. In this paper, the gill Na⁺-K⁺ATPase (NKA) activity and iono- and osmoregulation of juvenile R. canadum were investigated in a 12 h stress of ambient salinities (0-45), and the results showed that after an abrupt transfer to the salinities of 0, 5, 15, 25, 37 (control) and 45, the death of juvenile R.canadum only occurred in salinity 0, with a mortality of 100% by the end of the experiment. In all treatments, the gill NKA activity and serum osmolality fluctuated in first 3 h, and then changed smoothly. The NKA activity varied with salinity grade in U shape, being significantly (P<0.05)higher in salinity 5 and the lowest in salinity 15 in 12 h, while the serum osmolality (ranged 293-399 mOsmol·kg^-1) presented a positive correlation with salinity. Serum ［Na⁺］ and ［Cl^-］ concentration slightly increased with salinity within the period of 3-12 h, while serum ［K⁺］ displayed a reverse pattern. The isosmotic point was estimated as 328.2 mOsm·kg^-1 and corresponded to salinity 11.48. The isoionic points for serum ［Na⁺］, ［K⁺］ and ［Cl^-］ were estimated as 155.2, 6.16, and 137.1 mmol·L^-1, which corresponded to the salinities of 10.68, 20.44 and 8.41, respectively. It was concluded that R. canadum could be chharacterized physiologically as a “higher-NKA-in-hyposmotic media” marine euryhaline teleost with the capability of rapid and effective hyper/hypo iono- and osmoregulation.