S-layers are surface layers of bacterial cell walls. They are formed by two-dimensional, monomolecular crystalline arrays of identical units of protein or glycoprotein macromolecules (subunits). In general, each S-layer exhibits one of four possible 2-D lattice types: oblique (p1 or p2 symmetry), triangle (p3 symmetry), square (p4 symmetry) or hexagonal (p6 symmetry). The S-layer protein compasses up to 15% of the total protein of the bacterial cell and thus represents its major protein. Since 1972, S-layers have also been found in cyanobacteria. So far, they have been observed in 60 strains (isolates) of 23 species, belonging to 12 genera of unicellular Chroococcales and in just five strains or isolates (four species, four genera-only with p1 and p4 lattice symmetry) of filamentous Oscillatoriales; in further families of filamentous cyanobacteria (Nostocales, Stigonematales) they have not been detected, although filamentous cyanobacteria have been frequently studied in the electron microscope. In Chroococcales, relatively large cells of planktonic genera harbouring gas vesicles, S-layers are often present, while picoplanktonic species without gas vesicles usually do not have them. The p6 lattice symmetry appears to be the most common in cyanobacteria, having been found in 41 out of the 60 S-layers observed. All cells of a given strain, all strains capable of forming S-layers and all S-layer forming species of a given genus (as far as it is known) form S-layers of the same lattice type. Hence, the ability to form an S-layer appears to be useful as a supportive morphological marker for species classification. In 41 S-layer formers, the center-to-center spacing of their lattice unit arrays has been measured; the lattice constants range from 5 to 22nm, measured directly on surface of fixed cells. Coarse S-layers of p6 symmetry are the most frequent (with spacing of 15.0-22.0nm); p1 and p2 S-layers are the finest ones (with spacing of 5.0-10.0nm). Medium-spaced lattices (11.0-14.0nm) may be both of the p4 or p6 symmetry types. When measured on isolated S-layers, the spacings show a 10-60% higher value. All the hexagonal unit lattices have the same molecular architecture. Each S-layer unit resembles a truncated cone with an axial pore and with six protein subunits symmetrically placed around its opening. Adjoining units are interspaced by relatively fine channels. The fine detail of every S-layer of every individual strain is unique. Only the S-layer protein subunits of Synechococcus sp. strain GL24 have been analysed by electrophoresis. When incorporated into the S-layer units they confer a net neutral charge to the cell surface. This cyanobacterium induces mineralization of fine-grain gypsum and calcite in a saturated lake fresh water solution. This process is involved in the formation of stromatolites.