The morphology of lipopolysaccharides isolated from smooth and rough strains of Salmonella typhimurium was studied by electron microscopy using a positives-taining procedure. When viewed face on, lipopolysaccharide from the smooth strain was a homogeneously stained ribbon which could be broken down to homogeneously stained discs by sonication or agitation with ether. When viewed edge on, the ribbon and disc had a trilaminar appearance reminiscent of cross-sections of arrays of bimolecular leaflets. Lipopolysaccharide from a mutant lacking O-polysaccharide side-chains had a similar appearance and identical dimensions, whereas that from a heptoseless mutant, lacking O and “core” polysaccharide had smaller and more delicate trilaminar structures. The behavior of lipopolysaccharides in lipid solvent and the common morphological feature found in lipopolysaccharide regardless of the polysaccharide content suggest that the major morphological determinant is the lipid moiety. There is suggestive evidence that the “core” polysaccharide contributes to the trilaminar structures, whereas the O-polysaccharide side-chains do not.