A synthetic phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), was studied by differential scanning calorimetry at different hydration levels. A glass transition was observed in the lamellar gel (L-beta) phase but was not detected in a more ordered lamellar crystalline phase. The glassy state of the L-beta phase of POPE has properties that are typical of other glass-forming materials, i.e., nonexponential relaxation behavior below the glass transition temperature, a broad distribution of relaxation times, and plasticization by water. The significance of the glass transition in biological membranes is discussed.