Salmonella enterica is highly polymorphic for the O antigen, a surface polysaccharide that is subject to intense selection by the host immune system. This polymorphism is used for serotyping Salmonella isolates. The genes encoding O antigen biosynthesis are located in the rfb gene cluster. We report here the cloning and sequence of the 19-kb rfb region from strain M32 (serovar anatum, group E1) and compare it with that of strain LT2 (serovar typhimurium, group B). Genes for biosynthetic pathways common to both strains are conserved and have very similar sequences. In contrast, the five genes for CDP-abequose synthesis, present in strain LT2, are absent in strain M32; three open reading frames (ORFs) of strain LT2, thought to include genes for transferases, are not present in strain M32 but are replaced by three different ORFs with little or low level of similarity. Both rfb gene clusters are low in G + C content, indicating that they were transferred from a common ancestral species with low G + C content to S. enterica relatively recently (in the evolutionary sense). We discuss the recombination and lateral transfer events which may have been involved in the evolution of the polymorphism.