P. multocida, one of many species of Pasteurella in the respiratory flora of animals, is acause of respiratory infection in some individuals. This small, coccobacillary, Gram-nega-tive organism grows readily on blood agar but not on MacConkey agar. It is oxidase posi-tive and ferments a variety of carbohydrates. Unlike most Gram-negative rods, P. multo-cida is susceptible to penicillin. Humans are usually infected by the bite or scratch of adomestic dog or cat. Infection develops at the site of the lesion, often within 24 hours. The typical infection is a diffuse cellulitis with a well-defined erythematous border. The diag-nosis is made by culture of an aspirate of pus expressed from the lesion. Frequently, too few organisms are present to be seen on a direct Gram smear. P. multocida is by far the most common cause of an infected dog or cat bite. For unknown reasons, P. multocida is occasionally isolated from the sputum of patients with bronchiectasis. Infections are treated with penicillin.