Isolates of Bacteroides fragilis from 83 infected patients were collected during 2 years. Of 103 isolates, 68% were subspecies fragilis. This was the single most common subspecies isolated from blood (12/17), intra-abdominal (11/28), wound (17/20), perirectal (11/16), pelvic (6/10), and other (8/11) sites. Sixteen patients had B. fragilis isolated in pure culture. Of these, 9/12 blood cultures and 4/4 cultures from other sites were ss. fragilis. The recent observation that ss. fragilis organisms are encapsulated whereas the other subspecies are generally unencapsulated was supported by indirect fluorescent antibody studies using antibodies prepared to the capsular material of ss. fragilis. All 33 strains of ss. fragilis and none of 13 strains of other subspecies were positive. The predominance of ss. fragilis in clinical isolates compared to its relative infrequency in normal fecal flora and its predominance in pure cultures, especially of blood, indicate that this is the most virulent of B. fragilis subspecies. This virulence may be related to encapsulation.