The effects of environmental Na+/K+ ratio on the gill ion-transport enzyme activity,plasma osmolality and growth of Cynoglossus semilaevis juveniles were investigated. The results showed that,plasma osmolality was similar among flounder adapted to different Na+/K+ ratios of saline groundwaters (P>0.05),while the growth,gill Na+,K+-ATPase and HCO3--ATPase activities were affected by Na+/K+ ratio significantly (P<0.05). The gill Na+,K+-ATPase activity reached its maximum on day 3,then decreased gradually from day 3 to day 9 and remained constant from day 9 to day 15. The peak values of gill Na+,K+-ATPase activity were detected on day 3 for all Na+/K+ ratios of saline groundwaters,then the enzyme activities descended,and on day 9 the enzyme activi-ties achieved steady state,and the gill HCO3--ATPase activity increased rapidly and achieved steady state after one day. During steady state,the gill Na+,K+-ATPase and HCO3--ATPase activity of Na+/K+ ratios 20 and 40 treatments were significantly higher than those in the control group (Na+/K+ ratio 27.5),while there were no significant differences between the Na+/K+ ratio 30 treatment and the control group; the gill Na+,K+-ATPase activity of Na+/K+ ratio 20 and 40 treatments were significantly higher than that for ratio 30 treatment,but there were no significant differences of gill HCO3--ATPase activity among these treatments. At the end of the 15-day experiment,the weight gain (%) and specific growth rate (SGR) of flounders maintained in seawater were significantly higher than those in groundwaters; significant differences also occurred among the treatments; Na+/K+ ratio 30 treatment had the highest values (33.7% and 1.94 respectively),which were significantly higher than those under Na+/K+ ratios 20 and 40 treatments. There-fore,for the saline groundwater used in this experiment,it is suggested that the Na+/K+ ratio be adjusted to approximately 30,i.e.,as close to that of natural seawater as possible in the culture of flounder. The effects of environmental Na + / K + ratio on the gill ion-transport enzyme activity, plasma osmolality and growth of Cynoglossus semilaevis juveniles were investigated. The results showed that, plasma osmolality was similar among flounder adapted to different Na + / K + ratios of saline groundwaters P <0.05), while the growth, gill Na +, K + -ATPase and HCO3 - ATPase activities were significantly increased by Na + / K + ratio (P <0.05) The peak values ​​of gill Na +, K + -ATPase activity were detected on day 3 for all Na + / K + ratios of saline groundwaters, then the enzyme activities descended , and on day 9 the enzyme activi-ties achieved steady state, and the gill HCO3 - ATPase activity increased rapidly and achieved steady state after one day. During steady state, the gill Na +, K + -ATPase and HCO3 - ATPase activity of Na + / K + ratios 20 and 40 treatments were significantl while Na + / K + ratio 27.5, while there was no significant differences between the Na + / K + ratio 30 treatment and the control group; the gill Na +, K + -ATPase activity of Na + / K + ratio 20 and 40 treatments were significantly higher than that for ratio 30 treatment, but there were no significant differences of gill HCO3 - ATPase activity among these treatments. At the end of the 15-day experiment, the weight gain (%) and specific growth rate SGR) of flounders maintained in seawater were significantly higher than those in groundwaters; significant differences also occurred among the treatments; Na + / K + ratio 30 treatment had the highest values ​​(33.7% and 1.94 respectively), which were significantly higher than those under Na + / K-times 20 and 40 treatments. There-fore, for the saline groundwater used in this experiment, it is suggested that the Na + / K + ratio be adjusted to approximately 30, ie, as close to that of natural seawater as possible in the culture of flounder.