Abstract
Tyzzer's disease (Bacillus pilifarmis infection) was diagnosed at necropsy in a hysterectomy-derived, gnotobiotically reared guinea pig. This is the first description of spontaneous vertical, presumably transplacental, transmission of B. pilifarmis infection. Subsequent horizontal transmission of the infection was concluded from the presence of specific anti-B. pilifarmis antibodies in the sera from guineapigs originating from different litters but reared in the same isolator.