SummaryThe three‐dimensional structures of the crystalline surface layers of two species ofEubacteriahave been determined by electron microscopy and computerized image processing. The S‐layer ofEubacteriumsp. ES4C has tetragonal symmetry, with a unit cell spacing of 10.6 nm and a thickness of 9.5 nm, while that ofEubacteriumsp. AHN 990 has hexagonal symmetry a = b = 15.7 nm and a thickness of 13 nm. The resolutions in the reconstructions were 2.5 nm and 1.8 nm, respectively. The reconstruction of the S‐layer of strain ES4C reveals a distinct domain structure: a major tetramer, arms connecting adjacent unit cells, and a minor tetramer. The S‐layer of strain AHN 990, on the other hand, has a rather complex arrangement, centred around the six‐