The transbilayer distribution of phospholipids in Bacillus megaterium is asymmetrical, with twice as much phosphatidylethanolamine internally as externally (Rothaman, J. E. & Kennedy, E. P. (1977) J. Mol. Biol. 110,603-618). We now report that the biosynthesis of phosphatidylethanolamine is also asymmetrical. Newly synthesized phosphatidylethanolamine was found first on the cytoplasmic side of the membrane of pulse-labeled cells and later was redistributed until the specific radioactivity of the outer face became equal to that of the inner face of the bilayer. The rate of transmembrane movement is at least 30,000 times faster than the rate of spontaneous diffusion (flip-flop) of phosphatidylethanolamine across artificial phospholipid bilayers, indicating that transmembrane movement must be a facilitated process in living cells, perhaps involving membrane proteins.