Following an epizootic of septicemic pasteurellosis in a herd of American bison in 1965, calves have been vaccinated each fall. There have been no further signs of the disease. Results of bacteriological and serological studies on this herd are reported. Pasteurella multocida and Pasteurella haemolytica were not isolated from nasal cavities. Antibodies against P. multocida serotypes 1 (pronghorn strain), 2 (vaccine strain), and 4 (black bear strain) were detected in serum samples from some bison. No antibodies were detected against serotype 3, a bovine strain originally isolated from the lung of a calf. All serum samples contained antibodies against parainfluenza-3 virus.