BACKGROUND We identified a motile, curved, gram-negative bacillus as the cause of persistent fever and bacteremia in two patients with symptomatic human immunodeficiency virus infection. The same organism was subsequently recovered from a bone marrow-transplant recipient with septicemia and from two immunocompetent persons with week-long febrile illnesses. All the patients recovered after antimicrobial therapy. RESULTS Primary cultures of blood processed by centrifugation after blood-cell lysis yielded adherent, white, iridescent, morphologically heterogeneous colonies in 5 to 15 days. Subcultures grew in four days on chocolate, charcoal-yeast extract, or blood agar. The organisms stained weakly with safranin and were not acid-fast. Fluorescent-antibody tests for legionella and francisella were negative. Biochemical reactivity was minimal and difficult to ascertain. Agar-dilution testing revealed in vitro susceptibility to most antimicrobial agents tested. The cellular fatty acid composition of the isolates was similar, resembling that of Rochalimaea quintana or brucella species, but not Helicobacter pylori or species of campylobacter or legionella. As resolved by gel electrophoresis, cell-membrane preparations of all isolates contained similar proteins, with patterns that differed from that of R. quintana. Patterns of digestion of DNA from all isolates by EcoRV restriction endonuclease were virtually identical and also differed from that of R. quintana. On immunodiffusion, serum from one convalescent patient produced a line of identity with sonicates of all five isolates. CONCLUSIONS This pathogen may have been unidentified until now because of its slow growth, broad susceptibility to antimicrobial agents, and possible requirement of blood-cell lysis for recovery in culture. It should be sought as a cause of unexplained fever, especially in persons with defective cell-mediated immunity.