The structure of a purified protein associated with the cell wall polysaccharides of the marine green algaeAcetabularia (Polyphysa) cliftonii has been studied by means of X-ray diffraction, infrared spectroscopy and circular dichroism. The homogeneous preparation of the cell wall protein has a molecular weight of 14,000, as determined by sodium-dodecylsulfate electrophoresis. Regular layer line reflections on the X-ray diffraction photographs suggest that a distinct order exists in the arrangement of the protein fibrils. Through infrared spectroscopy of thin aqueous films of the protein, as well as of the fibers, it was established that the α-helical structure is predominant in the cell wall protein. The fibers crystallize in a hexagonal unit cell witha=14.5 A and c=27.0 A, at a water content of two molecules per residue. Increase in water content causes an increase in thea-axis, but without change in thec-direction, thus keeping the α-helical conformation. Moreover the spectral data in the amide A, I, II, III, and IV-regions show that the cell wall protein has an ordered α-helical conformation.