The surface of a gram-negative bacterium is made up of the lipopolysaccharide (l.p.s.) and protein components of the outer leaflet of its outer membrane, and of capsular polysaccharide, flagella and fimbriae if present. In Salmonella all the special genes needed for synthesis of the O-specific oligosaccharide repeat unit (different in each O group) of the l.p.s. sidechains are found in the rfb cluster, near his. Nearly all so-far identified rfa genes, for synthesis of l.p.s. core, are clustered between cysE and pyrE. Genes for polymerization and modification of O units are scattered: some are part of prophage genomes and some show 'form variation' - spontaneous alternation between expression and non-expression, mechanism unknown. Escherichia coli differs by frequent presence of capsular polysaccharides (K antigens), some determined by kps genes, unlinked to l.p.s. genes, others by his-linked genes perhaps homologous with rfb. Expression of some non-l.p.s. polysaccharide genes, but not of l.p.s. genes, is greatly influenced by the environment. Major outer membrane proteins (more than 10$^{5}$ molec./bacterium) include: a lipoprotein, in part covalently joined to the cell wall, perhaps anchoring the outer membrane; and several proteins of molec. mass 30 000-40 000 (one of them phage-determined), some of which serve to make the outer membrane permeable to small hydrophilic molecules. Genes affecting sensitivity (adsorbing capacity) to various phages and colicins (e.g. tonA, bfe) specify various 'minor' outer membrane proteins concerned with uptake of nutrients (e.g. iron ferrichrome, vitamin B$_{12}$) when present at very low concentrations. Neither the 'major' nor the 'minor' protein genes are clustered: their expression is subject to conspicuous regulation by environmental conditions. In E. coli the flagellin and hook protein structural genes are located in different clusters of motility-related genes. Missense mutations in the flagellin gene may cause alteration in flagellar shape or in serological character, which in Salmonella is also affected by gene nml, for methylation of the free amino groups of some lysines of flagellin. Electron microscopy of re-annealed DNA from the relevant region indicates that change of flagellar antigenic phase in Salmonella results from a reversible inversion of a 750 base-pair segment, probably constituting the phase-determinant gene. Production of fimbriae (pili) requires function of several linked pil genes, and is subject to a kind of 'form variation' of unknown mechanism. Genes in conjugative plasmids when derepressed cause production of sex pili. E. coli protein antigens K88 and K99, apparently fimbrial, concerned with adhesion to intestinal mucosa and so with enteropathogenicity, are plasmid-determined.