Crystalline features of cellulose microfibrils in the cell walls of Glaucocystis (Glaucophyta) were studied by combined spectroscopy and diffraction techniques, and the results were compared with those of Oocystis (Chlorophyta). Although these algae are grouped into two different classes, by the composition of their chloroplasts for instance, their cell walls are quite similar in size and morphology. The most striking features of their cellulose crystallites are that they have the highest cellulose I-alpha contents reported to date. In particular, the I-alpha fraction of cellulose from Glaucocystis was found to be as high as 90% from C-13 NMR analysis. The mode of preferential orientation of cellulose crystallites in their cell walls is also interesting equatorial 0.53-nm lattice planes were oriented parallel to the cell surface in the case of Glaucocystis, while the 0.62-nm planes were parallel to the Oocystis cell surface. Such a structural variation provides another link to the! evolution of cellulose structure, biosynthesis, and its biocrystallization mechanism,