The membrane pattern of the Tetrahymena cortex is investigated using thin-sectioning and freeze-etching electron microscopy. The plasma and alveolar membranes are different with respect to their thin-sectioning and freeze-etching appearance. Freeze-etching reveals new structural elements, i.e. specific arrangements of particles differently sized and differently disposed in the pellicular, ciliary, and mucocyst membranes. These membrane-associated particles apparently correlated to specific organelles are discussed with respect to their functions.