The results of an investigation by X-ray diffraction on the supramolecular organization of a peculiar class of semisynthetic lipids, namely bipolar lipids from archaebacteria, when organized in layered structures in Langmuir-Blodgett films are reported. The simplicity of the experimental technique and the straightforward interpretation of the data provide a clear and unambiguous determination of the molecular conformation of bipolar lipids in LB films. The experimental data make it possible to reject unequivocally both the model of monolayers of fully extended molecules and the model of monolayers of tilted molecules, while they strongly support a structural model consisting of the stacking of bilayers made of bent U-shaped molecules. The results reported may constitute a direct clarifying contribution to the controversial matter of the molecular conformation of bipolar lipids at the air-water interface.