Rhizobia are soil bacteria typically able to symbiotically interact with legume plants to produce nitrogen fixing root nodules. While all rhizobia were once placed under the same genus, Rhizobium, nowadays the classification of rhizobia using molecular analysis has revealed a more complex relationship among strains. Several genera have been described and accepted (i.e. Rhizobium, Allorhizobium, Azorhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium) with many species in each genus. However, within the same species, it is common to observe different strains with quite different physiological and biochemical profiles. While some traits are common among all rhizobia selected traits such as denitrification seem to be randomly distributed among genera and species. Moreover, many species are true denitrifiers (strains of Bradyrhizobium japonicum) while some are only partial NOx-reducers (strains of Rhizobium sullae) (Toffanin et al., 1996; Squartini et al. 2002). While the evolutionary advantage deriving from the complete denitrification pathways may be easily explained, the advantage gained from expressing only a fragment of such a metabolic property has not been completely clarified (Toffanin et al., 2000). A comparison will be made among rhizobia showing these traits, taking into account both free and symbiotic forms