In two patients with prosthetic valve endocarditis due to Peptostreptococcus magnus, blood cultures in the BacT/Alert and BACTEC 9240 systems were signal negative. The capability of the BacT/Alert system to detect various Peptostreptococcus species Ras assessed. P. magnus and P. anaerobius could not be detected, and subcultures remained negative. The growth in conventional media of these two species and other Peptostreptoroccus species was similar.