One of the most challenging questions that relates to the structure and function of biological membranes is whether the two halves of the bilayer "talk" to each other. In this letter, we show how the pertubation of the lateral organization of one leaflet of a fluid phospholipid bilayer by an external agent also alters the lateral organization of the adjoining leaflet. In addition, we show that the energy involved in such "cross talk" corresponds to ca. 100 cal/mol of phospholipid. These findings provide a basis for expecting similar cross talk to exist in cell membranes.