Transduction, bacteriophage-mediated gene transfer, is thought to play an important role in the evolution of prokaryote genomes. Several gene transfer agents that resemble transducing phages have been found in diverse prokaryotes. This mini-review discusses these interesting agents of genetic exchange with a focus on the gene transfer agent (GTA) of Rhodobacter capsulatus, at present the only member of this group for which genetic information exists about the production of transducing particles. Production of GTA results from expression of genes that are similar to phage genes, yet transcription of these genes is dependent upon cellular (two-component) signaling proteins. The significance of these relationships, as well as the finding of GTA gene homologues in the bacterium Rhodopseudomonas palustris, is discussed.