Haemophilus aphrophilus is a facultative anaerobic, gram-negative coccobacillus or bacillus and its growth is stimulated by 5 to 10% CO2 . Most Haemophilus species require either exogenous X or V factor or both to grow, but H. aphrophilus can grow without these factors. H. aphrophilus rarely causes invasive infections such as endocarditis, septicemia, pneumonia and peritonitis in human. Two cases of infective endocarditis by H. aphrophilus have been reported in Korea. However, there has been no report of polymicrobial endocarditis by H. aphrophilus and other bacteria. We isolated H. aphrophilus and coagulase-negative staphylococci (CNS) from the blood of a 38-year-old woman with prosthetic valve endocarditis. She underwent an emergent operation and a culture of the prosthetic valve grew H. aphrophilus. Brain abscess was developed at hospital day 11. H. aphrophilus was susceptible to all antibiotics tested such as ampicillin and cefotaxime, and CNS was susceptible to oxacillin and vancomycin. The patient responded well to therapy with ceftriaxone, teicoplanin, and gentamicin. (Korean J Clin Microbiol 2006;9(1):71-75)