The 5s ribosomal ribonucleic acid (rRNA) sequences were determined for Bacteroides fragilis, Bacteroides thetaiotaomicron, Bacteroides capillosus, Bacteroides veroralis, Porphyromonas gingivalis, Anaerorhabdus furcosus, Fusobacterium nucleatum, Fusobacterium mortiferum, and Fusobacterium varium. A dendrogram constructed by a clustering algorithm from these sequences, which were aligned with all other hitherto known eubacterial5S rRNA sequences, showed differences as well as similarities with respect to results derived from 16s rRNA analyses. In the 5s rRNA dendrogram, Bacteroides clustered together with Cytophaga and Fusobacterium, as in 16s rRNA analyses. Intraphylum relationships deduced from 5s rRNAs suggested that Bacteroides is specifically related to Cytophaga rather than to Fusobacterium, as was suggested by 16s rRNA analyses. Previous taxonomic considerations concerning the genus Bacteroides, based on biochemical and physiological data, were confirmed by the 5s rRNA sequence analysis.The genus Bacteroides as described by Holdeman et al. (12) consists of more than 40 species which vary considerably in physiological, biochemical, and chemical properties and possess deoxyribonucleic acid (DNA) base compositions within the range from 28 to 61 mol% guanine plus cytosine (G+C). It is now generally accepted that a DNA base composition difference of more than 10 mol% G+C implies that the species concerned are unrelated at the generic level, yet notable exceptions occur (25). On the basis of physiological and biochemical properties, such as peptidoglycan composition, acid end products, lipids, and fatty acids (for a review see reference 30), various proposals have been made to exclude certain species from the genus Bacteroides (3, 30). Many of these proposals are in good agreement with ribosomal ribonucleic acid (rRNA) homology data (17). Thus, many of the species listed in Bergey's Manual of Systematic Bacteriology (12) have now been given separate generic status (1, 2, 4, 23,28,29,[31][32][33]. Recently, it has been suggested (3) that only members of the so-called "Bacteroides fragilis group" (viz