Adhesion to mucosal cells is an important virulence attribute of bacterial pathogens colonizing these sites. Bacteria of the upper respiratory system, such as members of the genus Bordetella, have well-defined adhesins. The main adhesin of B. pertussis is the filamentous hemagglutinin which can be used by other bacteria for attachment. The main adhesin of B. bronchiseptica is the bovine erythrocyte hemagglutinin. In both Bordetella species the presence of fimbriae does not appear critical to adhesion. In contrast, atrophic rhinitis (AR)-producing strains of Pasteurella multocida colonize poorly the pig's nasal mucosa. We performed an in vitro trial using newborn pigs' turbinate explants and showed that two toxigenic strains (serotype D fimbria + and serotype A fimbria -) were adherent when observed by scanning electron microscopy (SEM). Intranasal inoculation of both six week old and newborn SPF pigs with various strains of P. multocida also resulted in colonization. Adhesion was best achieved by toxigenic strains, regardless of possession of fimbria, hemagglutinin or capsular serotype. Colonization was more abundant and constant in tonsils. Nasal colonization was sporadic and sparse. Colonization of trachea and lung was only observed with serotype A strains. The results showed that toxigenic P. multocida can colonize the upper respiratory tract, especially the tonsils, of pigs.