Bordetella bronchiseptica produced tracheobronchitis when administered in aerosol to specific pathogen-free dogs. Clinical signs appeared to be directly related to numbers of bacteria in the trachea. Electron microscopic examination revealed that each bacterium was close to one or more tracheal cilia and that a fibrillar material was radiating from the bacterial cell wall. B. bronchiseptica required 14 weeks to be cleared from the tracheas of infected dogs; in contrast, other organisms commonly isolated from the respiratory tracts of dogs were cleared within one to three days. Strains of high and low in vitro passage and strains representing three different morphotypes were of equal pathogenicity. Local immunity was observed after infection and appeared to be of primary importance in recovery from the infection. Presumably this response involves prevention of bacterial attachment and reattachment to cilia.