In this work, we discuss the physical properties of black lipid membranes formed from a single monolayer of bipolar lipids at the air-water interface. The lipid used was extracted from the thermophilic archaeon Sulfolobus solfataricus and is characterized by two different polar heads. Two classes of membrane can be formed from this compound. The first consists of membranes formed using a technique similar to that introduced by Mental and Mueller (Proc. Natl. Acad. Sci. USA, 69 (1972) 3561), although in this case the lipid is spread only on one side of the Teflon partition. The second consists of membranes formed by spreading the lipid on both sides of the Teflon partition. The conductance in the presence of valinomycin, current-voltage measurements and electroporation indicate that the first class bf membranes is asymmetrical, while the second is symmetrical. Models to account for the molecular arrangements are discussed.