The role in infection and the patterns of susceptibility to antimicrobial agents of 17 infrequently reported species of Bacteroides were studied. The findings of this research are summarized, and the literature dealing with recovery of these species from both the normal flora and infections of humans is reviewed. During a six-year period, 22% of 679 specimens from infections due to anaerobic bacteria that were studied by the authors yielded one or more of these "other Bacteroides species" (OBS). The most commonly isolated OBS were Bacteroides ruminicola subspecies brevis (63 patients), Bacteroides ureolyticus (38), Bacteroides bivius (26), and Bacteroides disiens (17). Appreciable resistance of some isolates to the beta-lactam antibiotics, erythromycin, and the tetracyclines was noted, whereas all strains were inhibited by chloramphenicol and metronidazole and almost all were inhibited by clindamycin. The data indicate that OBS are present in infection much more often than has been reported previously. The resistance of certain strains to some commonly used antimicrobial agents may necessitate a reassessment of the initial, empiric therapy of infections due to anaerobic bacteria.