Two bacteriophages (phi-Brb01 and phi-Brb02), lytic to Bacteroides ruminicola ss brevis AR20, were isolated from sewage water. Both phages possessed polyhedral heads and long noncontractile tails, and were classified as Siphoviridae of morphotype B1. Bacteria resistant to phages phi-Brb01 and phi-Brb02 arose following lysis of broth cultures. Survivors of phi-Brb01 infection were capsulated but remained susceptible to phi-Brb02 infection. Survivors of phi-Brb02 infection were noncapsulated and were resistant to attack by both phi-Brb01 and phi-Brb02. Neither phage lysogenized the host. Both phages contained double-stranded DNA, and their restriction endonuclease digestion patterns indicated that the phage genomes were circularly permuted and terminally redundant. Phage phi-Brb01 genome was examined in greater detail and confirmed to be circularly permuted, of size 33 kb, with a terminal redundancy of 2 kb, or 6% of the length of the genome. Circularly permuted genomes in phages of rumen bacteria do not appear to have been reported previously. At present, there is considerable interest in the genetic manipulation of rumen bacteria. The characterization of the phages described herein provides the basic information required for their use in the construction of vectors for the transfer of genetic material.