The ompA gene from Salmonella typhimurium, encoding a major heat-modifiable protein of the outer membrane, has been cloned and extensively characterized. When expressed in Escherichia coli the gene directs the synthesis of an OmpA protein which is functionally and topologically indistinguishable from that made in S. typhimurium, thus indicating that export and membrane incorporation are very similar in the two organisms. The S. typhimurium protein effectively substitutes for the E. coli polypeptide in F-dependent conjugation and in the uptake of certain colicins, although it cannot serve as the receptor for the OmpA-specific phages K3 and TuII. On examination of the primary sequence of the protein, predicted from the nucleotide sequence of its gene, it was found that those domains likely to be exposed on the cell surface were significantly different to the corresponding regions of the E. coli polypeptide. These differences in the structure of the two proteins have been used to interpret differences in their biological activities.