The phylogenetic relationships of all validly described species of the genus Xanthomonas and the type strain of Stenotrophomonas maltophilia were analyzed by sequencing and comparing 16S ribosomal DNAs (rDNAs). The two genera exhibited a mean sequence similarity value of 96.6%, corresponding to differences at 50 nucleotide positions on average. The species of the genus Xanthomonas exhibited relatively high levels of overall sequence similarity; the mean similarity value was 98.2%, which corresponds to an average of 14 mutual nucleotide differences. Within the genus Xanthomonas, a group containing Xanthomonas albilineans, Xanthomonas hyacinthi, Xanthomonas theicola, and Xanthomonas translucens clustered apart from the main Xanthomonas core, whereas Xanthomonas sacchari formed a third phylogenetic lineage. Due to the very restricted variability in 16S rDNA sequences within the genus Xanthomonas, rDNA signatures that have possible diagnostic value for differentiating the Xanthomonas species could not be determined with certainty. When sequence similarities were compared with DNA-DNA pairing data determined previously, there was only a limited correlation. This illustrates the different resolving powers of the techniques for determining phylogenetic hierarchies and for species delineation.