During the life cycle of the unicellular alga Cyanidium caldarium the surfaces of the plasmalemma and the adjacent cell wall develop a number of differentiated structures which can be demonstrated with the freeze-etching technique. While cell division takes place, the plasma membrane is undifferentiated and covered with randomly distributed 55 and 80 angstrom particles as well as small holes from torn out particles that can be found adhering to the adjacent cell wall. The 80 angstrom particles possess a substructure and sometimes 40 angstrom fibrils can be seen leading from these particles into the cell wall. Just after cell division, shallow depressions showing a hexagonal surface pattern with a spacing of 105 angstrom and arrays of approximately hexagonally packed 55 angstrom particles are formed on the plasmalemma. The corresponding structures found on the cell wall are particle-studded humps, which fit into the shallow depressions, and faintly striated regions, which match the 55 angstrom particle arrays. During the next stage of development, the hexagonally patterned shallow depressions on the plasma membrane are transformed into regularly striated 300 to 350 angstrom wide and approximately 250 angstrom deep folds, while the arrays of 55 angstrom particles increase in size. On the adjacent cell wall we can follow the development of the particle-studded humps into ridges covered with 70 angstrom particles. The plasmalemma of old mature cells is characterized by long striated folds that replace nearly all network structured depressions, and a few small arrays of 55 angstrom particles. Long ridges covered with particles are the corresponding dominant feature on the inside of the cell wall. Prior to cell division, the striated folds and the other differentiations of the plasmalemma are broken down and eventually disappear so that the cell has again an undifferentiated 'embryonic' plasma membrane for cell division. Simultaneously the differentiated structures on the cell wall disappear. All the described particles and units forming plasma membrane differentiations seem to be confined to the surface layer of the plasmalemma. The outlined development cycle of the plasmalemma of Cyanidium shows that biological membranes have the potential to differentiate in time and space.