The thermal stability of liposomes made from polar lipids of Sulfolobus acidocaldarius, a thermoacidophilic archaebacterium, was studied. While two analogous nonarchaebacterial liposomes examined were unstable above 50 degrees C, liposomes made of the native tetraether lipids showed stability up to at least 80 degrees C. Incubation of the archaebacterial liposomes at 100 degrees C, under slight pressure, caused an approximate 15% leakage after 60 minutes. Boiling, however, destroys the integrity of the lipid membranes. The tetraether lipid vesicles are less permeable to solute molecules, such as carboxyfluorescein, than the other lipid vesicles at any temperature. The permeability is also less temperature-sensitive than that for other lipids.