A review with 81 refs. Horizontal gene transfers among bacteria, such as natural transformation or conjugation, may have played an important role in bacterial evolution. They are thought to have been involved in promoting genome plasticity which permitted bacteria to adapt very efficiently to any change in their environment and to colonize a wide range of ecosystems. Evidence that some genes were transferred from eukaryotes, and in particular, from plants to vacteria, was optained from nucleotide and protein sequence analyses. However, numerous factors, including some which are endogenous to the vacterial cells, tend to limit the extent of transfer particularly among phylogenitically distant orgaisms. The goal of this paper is to give an overview of the potentials and limits of natural interkingdom gene transfers, with particular focus on prokaryote-originating sequences which fit the nuclear genome of transgenic plants.