Pseudomonas aeruginosa adheres to respiratory epithelial cells in a highly specific fashion. In order to study the role of P. aeruginosa polar pili in the adherence process we conducted a quantitative morphological electron microscopic examination of P. aeruginosa adherence to SO2 injured canine tracheal cells in vitro. A pilin lacking background strain of P. aeruginosa PAK (BLP2) was constructed using a gene replacement and it in turn was engineered to express either the pilin gene of P. aeruginosa PAO, PAK, or no pilin gene. After 30 minutes incubation of these bacterial strains with injured canine tracheal rings the P. aeruginosa strains expressing pili adhered quantitatively more to the injured tracheal cells than did the pili lacking strains. PAO bearing strains adhered in greater numbers than PAK bearing strains. Healthy tracheal cells did not have any bacteria bound to their surfaces. The bacteria bound to the cilia and lateral edge of the exfoliating tracheal cells. Invasion of tracheal cells by piliated P. aeruginosa bacteria and penetration into the submucosa was also demonstrated. These data confirm the role of pili as important adhesins to injured tracheal cells. The difference in the adherence characteristics of pilin types PAK versus PAO may relate to the differences in the primary structure of these two pilin molecules.