Seven-day-old gnotobiotic pigs were inoculated intranasally with Pasteurella multocida and euthanatized 2, 5, 9, and 14 days after inoculation. Tissues from the oropharynx and respiratory tract of pigs were cultured quantitatively and analyzed microscopically. Pigs remained afebrile and alert, except one that died of acute fibrinopurulent pneumonia. Pasteurella multocida was isolated in greatest numbers from the pharyngeal tonsils, but only in low numbers from turbinate, trachea, lung, spleen, and liver. Significant histologic changes were limited to the tonsil. Infected pigs developed mild tonsillitis with lymphocytic hyperplasia, and accumulation of cell debris and bacteria in crypts. Capsular antigens of P. multocida, identified on tissue sections with rabbit anti-capsular polysaccharide antibody and immunocytochemical reagents, were confined to the crypt lumen. Ultrastructurally, bacteria were free within crypt material or within phagosomes of macrophages or neutrophils. In a second experiment, 5-day-old pigs were infected with Streptococcus suis type 2, followed by toxigenic Pasteurella multocida at 7 days of age; one pig died of streptococcal septicemia. Pigs developed a mild tonsillitis, and both bacteria were cultured from the tonsillar crypts for up to 14 days after infection. These studies show that a toxigenic strain of Pasteurella multocida, which is a causative agent of atrophic rhinitis, can colonize the tonsil and respiratory tract of gnotobiotic pigs for up to 14 days. In addition, colonization can occur concurrently with Streptococcus suis type 2.