Despite their close phylogenetic relationship, Escherichia coli and Salmonella typhimurium were long considered as having distinct classes of transposable elements maintained by either host-related factors or very restricted gene exchange. In this study, genetically diverse collections of E. coli and S. typhimurium (subgroup I) were surveyed for the presence of several classes of insertion sequences by Southern blot analysis and the polymerase chain reaction. A majority of salmonellae contained IS1 or IS3, elements originally recovered from E. coli, while IS200, a Salmonella-specific element, was present in about 20% of the tested strains of E. coli. Based on restriction mapping, the extent of sequence divergence between copies of IS200 from E. coli and S. typhimurium is on the order of that observed in comparisons of chromosomally encoded genes from these taxa. This suggests that copies of IS200 have not been recently transferred between E. coli and S. typhimurium and that the element was present in the common ancestor to both species. IS200 is polymorphic within E. coli but homogeneous among isolates of S. typhimurium, providing evidence that these species might differ in their rates of transfer and turnover of insertion sequences.