Using X-ray diffraction at glancing angles, we find that in Langmuir-Blodgett films of lead stearate the lead atoms form plane arrays identical to one set of planes in a bulk crystal, but with little correlation between the planes. The resultin system of twodimensional layers is similar in its static structure to on ideal smectic-B liquid crystaL Amphiphilic molecules (such as membrane consrituems) consist of a hydrophilic "head group" and a hydrophobic "tail";if the latter makes the material insoluble it can be spread as a monomolecular film on the surface of water [ I] _ The pressure-area curves of these films show changes of slope *, but although there are many hypotheses ** concerning the nature of the structures and transitions in these two-dimensional systems, these hypotheses have not so far been successfully tested with diffraction probes. These monolayers can, however, be transferred to solid substrates using the Langmuir-Blodgett technique [4]. Repeated dippings build up multilayer structures, with the molecules aligned normal to the layers in the same direction within each layer and in opposite directions in adjacent layers. These films have recently been the focus x For an excellent survey, see ref. 121. * The o+inal hypotheses were made by Harkins and Copeland 131. For a discussion and other references, see ref. [Z].