Sinorhizobium sp. BL3 is a salt-tolerant strain that can fix atmospheric nitrogen in symbiosis with leguminous host plants under salt-stress conditions. Since cell membranes are the first barrier to environmental ch nge, it is interesting to explore the membrane proteins within this protective barrier under salt stress. The protein contents of membraneenriched fractions obtained from BL3 were analyzed by nanoflow liquid chromatography interfaced with electrospray ionization tandem mass spectrometry. A total of 105 membrane proteins were identified. These proteins could be classified into 17 functional categories, the two biggest of which were energy production and conv ersion, and proteins not in clusters of orthologous groups (COGS). In addition, a comparative analysis of membrane proteins between salt-stressed and non-stressed BL3 cells was conducted using a membrane enrichment method and off-line SCX fractionation coupled to nanoLC-MS/MS. These techniques would be useful for further comparative analysis of membrane proteins that function in the response to environmental stress.