It is commonly assumed that the horizontal transfer of most bacterial chromosomal genes is limited, in contrast to the frequent transfer observed for typical mobile genetic elements. However, this view has been recently challenged by the discovery of lateral transduction in Staphylococcus aureus, where temperate phages can drive the transfer of large chromosomal regions at extremely high frequencies. Here, we analyse previously published as well as new datasets to compare horizontal gene transfer rates mediated by different mechanisms in S. aureus and Salmonella enterica. We find that the horizontal transfer of core chromosomal genes via lateral transduction can be more efficient than the transfer of classical mobile genetic elements via conjugation or generalized transduction. These results raise questions about our definition of mobile genetic elements, and the potential roles played by lateral transduction in bacterial evolution. It is commonly thought that horizontal transfer of most bacterial chromosomal genes is limited, in comparison with the frequent transfer of mobile genetic elements. Humphrey et al. show that, actually, phage-mediated lateral transduction of core chromosomal genes can be more efficient than the transfer of mobile genetic elements via conjugation or generalized transduction.