Sixty-eight bacteriophages specific for nine species (DNA homology groups) of Bacteroides were isolated from sewage. Four distinct morphological types were isolated, three of which had not previously been described. Attempts to use these phages to transduce Bacteroides fragilis were unsuccessful. A total of 91 phage-susceptible strains of Bacteroides were used with these phages in a study of the feasibility of developing a scheme for identification of Bacteroides species. Blind trials were performed with 10 B. fragilis-specific phages and a collection of over 200 bacterial strains, including 144 strains of R. fragilis. Of the strains of B. fragilis, 78% were identified correctly within 24 hr. A phage-carrier state, or pseudolysogeny, was observed with most of the phage-host systems, and this state was studied in detail with B. fragilis phage Bf-I. The presence of a thick capsule around some cells in a pure culture of a host strain appears to render these cells resistant to phage infection, thus perpetuating the carrier state. It is suggested that such capsules may play a role in the virulence of strains of Bacteroides.