The swimming crab, Portunus trituberculatus,is an important marine fishery and aquaculture species.Water salinity conditions influence its artificial propagations.In order to understand its adaptation mechanisms to environmental salinity changes,we investigated gill Na⁺/K⁺-ATPase activity in the P.trituberculatus exposed to different salinity stresses.Forty-five male adult crabs were divided into 3 groups(15 crabs for one group)and acclimated to three different salinity conditions(10,25 or 40).The 2^nd and the 6^th gills of three crabs from each treatment were sampled on days 0.5,1,2,3 and 5,respectively.In order to compare the Na⁺/K⁺-ATPase activities among different gills of a crab,we sampled all the eight gills from three crabs from each salinity group on day 1.Na⁺/K⁺-ATPase activities were measured by Molybdenum blue spectrophotometric method.Data were expressed as mean and standard error(mean±SD)and one-way ANOVA was used for statistical analysis.The results indicated that the activities of gill Na⁺/K⁺-ATPase in three salinity treatments decreased in the primary period,increased gradually,decreased again and reached a plateau finally.In addition,the Na⁺/K⁺-ATPase activities of the three posterior pairs of gills were remarkably higher than those of the five anterior pairs of gills.Based on these results,we proposed to divide osmotic regulation into three phases:passive stress phase(the decrease of the Na⁺/K⁺-ATPase activities),positive regulation phase(the elevation of the Na⁺/K⁺-ATPase activities)and adaptive phase(the stability of the Na⁺/K⁺-ATPase activities).The three posterior pairs of gills were the main sites of ion transportation and osmotic regulation in P.trituberculatus.This paper revealed the regulations of Na⁺/K⁺-ATPase activities of P.trituberculatus during salinity stress,and facilitated the understanding of physiological and salinity adaptative mechanisms in this species.