WORK on optical birefringence1, X-ray diffraction2,3 and, with reservations, electron microscopy of stained and dried particles4,5 indicates that phospholipids in coarse aqueous dispersion, prepared by shaking the phospholipid with water, have a multi-lamellar structure composed of alternate layers of phospholipid and the solvent. The phospholipid lamellae are thought to be molecular bilayers with hydrophilic headgroups on both surfaces and hydrocarbon chains directed towards the centre. When finer dispersions are required it is customary to irradiate the course dispersion with ultrasound, and the purpose here is to comment on the changes which occur during this treatment. © 1969 Nature Publishing Group.