A bstract Na + /K +-ATPases are membrane-associated enzymes responsible for the active transport of Na + and K + ions across cell membranes, generating chemical and electrical gradients. These enzymes’ α-subunit provides catalytic function, binding and hydrolyzing ATP, and itself becoming phosphorylated during the transport cycle. In this study, Na+ /K +-ATPase α-subunit c DNA was cloned from gill tissue of the swimming crab P ortunus trituberculatus by reverse-transcription polymerase chain reaction(RT-PCR) and rapid amplification of c DNA end methods. Analysis of the nucleotide sequence revealed that the c DNA had a full-length of 3 833 base pairs(bp), with an open reading frame of 3 120 bp, 5’ untranslated region(UTR) of 317 bp, and 3’ UTR of 396 bp. The sequence encoded a 1 039 amino acid protein with a predicted molecular weight of 115.57 k Da and with estimated p I of 5.21. It was predicted here to possess all expected features of Na+ /K +-ATPase members, including eight transmembrane domains, putative ATP-binding site, and phosphorylation site. Comparison of amino acid sequences showed that the P. trituberculatus α-subunit possessed an overall identity of 75%–99% to that of other organisms. Phylogenetic analysis revealed that this α-subunit was in the same category as those of crustaceans. Quantitative real-time RT-PCR analysis indicated that this α-subunit’s transcript were most highly expressed in gill and lowest in muscle. RT-PCR analysis also revealed that α-subunit expression in crab gill decreased after 2 and 6 h, but increased after 12, 24, 48, and 72 h. In addition, α-subunit expression in hepatopancreas of crab decreased after 2–72 h. These facts indicated that the crab’s Na+ /K +-ATPase α-subunit was potentially involved in the observed acute response to low salinity stress. A bstract Na + / K + -ATPases are membrane-associated enzymes responsible for the active transport of Na + and K + ions across cell membranes, generating chemical and electrical gradients. These enzymes’ alpha-subunit provides catalytic function, binding and hydrolyzing ATP , and itself becoming phosphorylated during the transport cycle. In this study, Na + / K + -ATPase α-subunit c DNA was cloned from gill tissue of the swimming crab Portunus trituberculatus by reverse-transcription polymerase chain reaction (RT-PCR) and Rapid amplification of c DNA end methods. Analysis of the nucleotide sequence revealed that the c DNA had a full-length of 3 833 base pairs (bp), with an open reading frame of 3 120 bp, 5 ’untranslated region (UTR) of 317 bp, and 3 ’UTR of 396 bp. The sequence encoded a 1 039 amino acid protein with a predicted molecular weight of 115.57 kDa and with estimated pI of 5.21. It was predicted here to possess all expected features of Na + / K + -ATPase members, including eight transm embrane domains, putative ATP-binding site, and phosphorylation site. Comparison of amino acid sequences showed that the P. trituberculatus α-subunit possessed an overall identity of 75% -99% to that of other organisms. Phylogenetic analysis revealed that this α- subunit was in the same category as those of crustaceans. Quantitative real-time RT-PCR analysis indicated that this α-subunit’s transcript were most highly expressed in gill and lowest in muscle. RT-PCR analysis also showed that α-subunit expression in crab In addition, α-subunit expression in hepatopancreas of crab decreased after 2-72 h. These facts indicate that the crab’s Na + / K + - ATPase α-subunit was potentially involved in the observed acute response to low salinity stress.