A patient with Waldenström's macroglobulinemia was treated empirically with imipenem for sepsis related to oropharyngeal infection and responded within 24 hours. When blood cultures yielded Streptococcus agalactiae, the regimen was changed to ampicillin and gentamicin. The patient's condition rapidly deteriorated, and she died 3 days later. After her death, a strain of Fusobacterium nucleatum subspecies polymorphum producing beta-lactamase (PEN-Y; group 2a) was isolated from blood cultures. A literature review revealed increasingly frequent isolation of beta-lactamase-producing strains of F. nucleatum. Thus strains of F. nucleatum isolated from blood and other specimens from patients with serious infections should be tested for beta-lactamase production.