Thermus thermophilus HB8 is a Gram-negative thermophilic eubacteria, with a well-defined outer membrane and a thin peptidoglycan layer around the cytoplasmic membrane. There is a S-layer on the cell envelope, whose main component is a protein of Mr around 100.000, masked by a lipopolysaccharide-like outer surface that can be revealed by freeze-fracture and shadowing of the native surface of the cells of T. thermophilus.When membrane fractions of this bacteria are treated with Triton X-100 in presence of EDTA, two-dimensional crystals with a hexagonal ordering could be observed by negative staining (figures 1a, 1b). This structure, called S1, had an unit cell dimensions a = b = 20 nm. Nevertheless, if the treatment is developed without the quelant agent, a structure with p2 symmetry is observed, S2, (figures 1d, 1e) with a = 24 and b = 21 nm. We have characterized both crystals by electron microscopy and two-dimensional analysis by Fourier methods. The resulting images showed very different characteristics; while the p6 structure is a typical S-layer, fig. 1c, the other one is a porin-like or regular outer membrane protein structure, fig. 1f, (2).This is surprising because both fractions contains the same ˝P100˝ protein (revealed by biochemical analysis).