Deoxyribonucleic acid (DNA)-ribosomal ribonucleic acid (rRNA) hybridizations were carried out between 23s 14C-or 3H-labeled rRNAs from type strains Pseudomonas jluorescens ATCC 13525, Pseudomonas acidovorans ATCC 15668, Pseudomonas solanacearum ATCC 11696, and Xanthomonas campestris NCPPB 528 and DNA from one or more strains of several taxonomically assigned or unassigned phytopathogenic Pseudomonas species. The strains were assigned to one of the following rRNA branches: (i) the P. Jluorescens rRNA branch (the authentic pseudomonads), containing also Pseudomonas agarici, Pseudomonas amygdali, Pseudomonas asplenii, Pseudomonas caricapapayae, Pseudomonas cichorii, Pseudomonas corrugata, Pseudomonas fuscovaginae, Pseudomonas marginalis (P. marginalis pathovar marginalis, P . marginalis pathovar alfalfae, and P. marginalis pathovar pastinacae) , Pseudomonas meliae, the Pseudomonas syringae group (including "Pseudomonas aceris, " "Pseudomonas antirrhini," "Pseudomonas apii,' ' "Pseudomonas berberidis," "Pseudomonas cannabina," 'LPseudomonas coronafaciens", "Pseudomonas eriobotryae," "Pseudomonas lapsa,') "Pseudomonas maculicola," "Pseudomonas oryzicola,9' "Pseudomonas panici," "Pseudomonas passiflorae," "Pseudomonas primulae," "Pseudomonas ribicolu," "Pseudomonas sesami," L'Pseudomonas striafaciens, )' P. syringae, "Pseudomonas ulmi,' ' and "Pseudomonas viburni"), Pseudomonas toluasii, Pseudomonas viridiflava, and "Pseudomonas washingtoniae;" (ii) the P . solanacearum rRNA branch, containing P. Additional approaches have been used to clarify the relationships among the taxa in the genus Pseudomonas. In general, deoxyribonucleic acid (DNA) base compositions (mean molar percent guanine plus cytosine [G+CJ) (46) and the results of DNA-DNA hybridization (58) agreed with the phenotypic classification scheme. DNA-ribosomal ribonucleic acid (rRNA) hybridization (59) revealed the existence of five rRNA homology groups of named Pseudomonas. These findings were supported by the results of immunological characterization of the host factor required for coliphage QP ribonucleic acid replication (24), the results of cellular fatty acid composition studies (36), and enzymic data (15).Recently, De Vos and De Ley (22) confirmed the existence of the same five rRNA groups by using another DNA-rRNA hybridization technique. However, by including many more strains and in particular many strains of a great variety of gram-negative taxa, these authors found that