An additional case ofBacteroides sp.endocarditis is reported. This rare infection may go undetected because of the fastidious growth requirements of the organism. Typically, it occurs in males above the age of 40 with underlying heart disease. The oropharynx, the gastrointestinal tract, and the genitourinary tract are the principal portals of entry. The clinical manifestations do not differ appreciably from those described for endocarditis due to facultative microorganisms except for an increased incidence of thrombophlebitis and embolic phenomena. The mortality rate of 36 per cent is in part the result of the penicillin resistance ofB. fragilis.Of the antibiotics currently available, chloramphenicol appears to be the drug of choice.