SummaryThe nucieotide sequence of the traT gene present in the virulence‐associated pfasmid ofSalmonella typhimuriumwas determined. The predicted TraT protein encoded by this gene was found to consist of 243 amino acids and to resemble the known TraT proteins of the plasmids of the F incompatibility group. Thus it contains a signal sequence of 20 amino acids, an amino‐terminal lipid attachment site, and two strongly hydrophobic regions close to each other in the mature protein. A mutation leading to increased permeability of the outer membrane to hydrophobic agents, previousfy focalrzed to thetraTgene, was shown to change a glycine residue to arginine within one of these hydrophobic regions. The same principle was found to apply to TraT of R6‐5: the introduction, by site‐directed mutagenesis, of either positively or negatively charged amino acids or the helix‐disrupting proline in the corresponding hydrophobic region led to increased hydrophobic permeability of the outer