We have studied the surface layer (S-layer) of Halobacterium salinarum (formerly Halobacterium halobium), an extreme halophile requiring high concentrations of sodium, by electron microscopy of (a) isolated, negatively stained, flattened envelopes and (b) cryo-fixation of intact cells in their high-salt growth medium followed by freeze substitution and tomography of thin sections. From the negatively stained isolated envelopes we have calculated a two-dimensional, projection map that is strikingly similar to that of Haloferax volcanii, an extreme halophile requiring high concentrations of magnesium; both projection maps show the hexagonal arrangement of the morphological units with an identical center-to-center spacing of 150 Angstrom; each of the morphological units of the tn species has six subunits with a similar density distribution and apparent domain organization. In contrast to the two-dimensional map, the tomographic reconstruction of Halob. salinarum does not agree in a straight-forward way with the three-dimensional, electron crystallographic map of negatively stained Halof. volcanii envelopes, although the main features of the lattice and the morphological units are evident. The tomographic reconstruction of sections from epoxy-embedded material suffers from directional compression due to sectioning stress and continuous dimensional changes and mass loss due to electron irradiation. This communication consists, therefore, of three parts: (a) a comparison of the projection maps of negatively stained envelopes of Halof: volcanii and Halob. salinarum; (b) a comparison of the three-dimensional maps obtained by electron crystallography (Halof. volcanii) and low-dose cryotomography (Halob. salinarum); and (c) a methodological study of mass loss and dimensional changes of plastic-embedded material under low-dose conditions at room and Liquid nitrogen temperatures.