SUMMARY: The relatedness between several nomen species of the group and some other organisms was numerically fixed through DNA homology and DNA base composition. For Pseudomonas the numerically analysed strains proposed by Lysenko as neotypes were used. The mean % (G+C) was in the range 60-67.5. C-DNA from either or was hybridized with DNA from 17 different species centres and the DNA homology was in the range 50-100%. Genetic species differentiation in the genus seems justified. In three border cases ( and ) DNA homology was only 28-50%, so the inclusion of these organisms in the genus is uncertain. The species centres and are omitted from the genus because of their very low DNA homology and aberrant DNA base composition. Twenty-eight nomen species of all form a narrow group in the range 63.5-69% (G+C). With two exceptions DNA homology with a median strain was always over 75% and frequently nearly complete. C-DNA from hybridized with -DNA to the same extent as with the pseudomonads proper. The cluster overlapped perfectly with part of the group. It is proposed therefore to gather all xanthomonads in a single genetic species This is such a dense cluster that the preservation of separate species names for the border cases seems undesirable. About one half to two-thirds of and -DNA is identical. The genera and appeared to be rather closely related to since they shared some 40-50% of their DNA. The genus appeared to be more closely related to than to The genera and shared some 20-30% DNA with but -DNA was almost entirely different. From a comparison between DNA homology and taximetric similarity, it appeared that most pseudomonads would not contain unused genes. The advantages of a classification based on % (G+C) and DNA homology are obvious.


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