Members of the genus Bdellovibrio possess the unifying phenotypic trait of attacking and preying upon other Gram-negative bacteria. It has been suggested that this common lifestyle arose by convergent evolution. Physiological and G + C studies have led to the notion that bdellovibrios are a heterogeneous group of loosely related bacteria. We have inferred the phylogenetic relatedness of 12 strains of Bdellovibrio through the analysis of partial 16S ribosomal RNA sequences. Similarity and degree of homology were assessed, and a phylogenetic tree was constructed by the distance matrix method. One branch of the two-branched tree consisted of B. bacteriovorus and related strains (W, 6-5-S, 109, 109D, 109J, 114, HI Ox9-2, and HI Ox9-3). The other branch was itself branched, with B. starrii, B. stolpii, and marine strain BM4 in separate sub-branches. All Bdellovibrio strains in turn clustered with representatives of the delta division of the Proteobacteria. The results indicate that there are at least two subdivisions of the genus Bdellovibrio and that present-day bdellovibrios arose from a common ancestor. The placement of the genus Bdellovibrio within the delta division of the Proteobacteria was confirmed.