The importance of Pasteurella multocida as an animal pathogen has been appreciated for many years. Its host range is remarkably broad. In recent years in central Canada P. multocida has been iso- lated from cattle, swine, sheep, goats, dogs, cats, rabbits, chickens, turkeys, ducks, chinchilla, mink, nice and guinea pigs. Reports of isolations from human beings appear occasionally in the literature. The distribution of this important parasite is worldwide. P. multocida is harboured in the respiratory passages of a considerable proportion of apparently normal domesticated animals. Some indication of the carrier rate is given in Table 1. It is not surprising in view of the high incidence of carriers that P. multocida occurs so frequently in disease involving the respiratory system. In years past the isolation of P. multocida from a diseased organ or tissue was considered sufficient evidence to attribute to it a primary aetiological role. However, in view of the rapid progress of animal virology in recent years it has become necessary to reassess the role of P. multocida in animal disease. Much of the available information on pasteurella infections was based upon reports which were published several decades prior to the extensive study of virus infections of animals.