The membranes of extremely halophilic Archaea are characterized by the abundance of a diacidic phospholipid, archaetidylglycerol methylphosphate (PGP-Me), which accounts for 50 -80 mol% of the polar lipids, and by the absence of phospholipids with choline, ethanolamine, inositol, and serine head groups. These membranes are stable in concentrated 3-5 M NaCl solutions, whereas membranes of non-halophilic Archaea, which do not contain PGP-Me, are unstable and leaky under such conditions. By x-ray diffraction and vesicle permeability measurements, we demonstrate that PGP-Me contributes in an essential way to membrane stability in hypersaline environments. Large unilamellar vesicles (LUV) prepared from the polar lipids of extreme halophiles, Halobacterium halobium and Halobacterium salinarum, retain entrapped carboxyfluorescein and resist aggregation in the whole range 0 -4 M NaCl, similarly to LUV prepared from purified PGP-Me. By contrast, LUV made of polar lipid extracts from moderately halophilic and non-halophilic Archaea (Methanococcus jannaschii, Methanosarcina mazei, Methanobrevibacter smithii) are leaky and aggregate at high salt concentrations. However, adding PGP-Me to M. mazei lipids results in gradual enhancement of LUV stability, correlating with the PGP-Me content. The LUV data are substantiated by the x-ray results, which show that H. halobium and M. mazei lipids have dissimilar phase behavior and form different structures at high NaCl concentrations. H. halobium lipids maintain an expanded lamellar structure with spacing of 8.5-9 nm, which is stable up to at least 100°C in 2 M NaCl and up to ϳ60°C in 4 M NaCl. However, M. mazei lipids form non-lamellar structures, represented by the Pn3m cubic phase and the inverted hexagonal H II phase. From these data, the forces preventing membrane aggregation in halophilic Archaea appear to be steric repulsion, because of the large head group of PGP-Me, or possibly out-of-plane bilayer undulations, rather than electrostatic repulsion attributed to the doubly charged PGP-Me head group.Microbial growth occurs over a wide range of salt concentrations spanning the whole range from fresh water environments to NaCl concentrations of 3-5 M (1-4). To survive under conditions of high salinity, halophilic and halotolerant microorganisms have developed specific cell and cell membrane adaptations (5). Eubacteria and other halotolerant organisms use organic solutes from the surroundings or synthesize compatible osmolytes to balance the high external osmotic pressure (1, 6). Extremely halophilic Archaea maintain osmotic balance by accumulating salts through osmoregulation (4) and have adapted their membranes to have low proton and sodium permeability at high salt concentrations (7). The membrane destabilizing effects caused by NaCl may be compensated for by a shift in lipid composition; for example, in Vibrio costicola, by a decrease in non-bilayer-forming phosphatidylethanolamine and a corresponding increase in phosphatidylglycerol (8).Halophilic and halotolerant microorganis...