The recent determination of the genome sequence of three Frankia strains has highlighted the evolutionary forces that have shaped the genetic makeup of the actinorhizal symbionts and it has opened up many avenues of research. Instances of gene duplication, gene loss and gene acquisition through lateral transfer show that the three Frankia genomes are dynamic and have evolved as a function of their host characteristics and biogeography. No convincing nod gene cluster or significant symbiotic island could be discerned. All the genes presently known to be involved in the symbiosis (nif, hup1 and hup2, shc) are found spread over the genome in at least four clusters. The results will be discussed with emphasis on understanding the mechanisms underlying the interaction and link between evolutionary forces and ecological adaptation to different biotopes.