In order to study the dynamics of avian colibacillosis, commercial broiler chickens were inoculated with a pathogenic Escherichia coli strain (01:K1:H7) into the left caudal thoracic air sac. Chickens were euthanatized at different times from 3 to 48 hr postinoculation and examined for bacterial counts and macroscopic and microscopic lesions. The E. coli strain colonized the air sacs, lungs, and trachea and was recovered from blood and all tested extrarespiratory organs of inoculated birds. A gradual increase in bacterial counts in the trachea, lungs, air sacs, and liver was observed from 3 to 12 hr. Clinical signs and macroscopic lesions of colibacillosis were observed in all inoculated birds. Moderate to severe lesions of airsacculitis, pericarditis, perihepatitis, and splenic hypertrophy were observed. Microscopically, inflammatory cell infiltration, serious to fibrinous exudate, and cellular debris on serosal surfaces were present in the liver, spleen, and air sacs. In air sacs, heterophils were present in low numbers perivascularly 3 hr after inoculation and became more numerous by 24 hr postinoculation. Ultrastructurally, epithelial cells in the air sacs and in air capillary regions of the lung were swollen and vacuolated beginning at 3 hr postinoculation. Bacteria were adherent to and present within the epithelial cells at 3 hr postinoculation and were also seen in phagocytic cells and, rarely, in the connective tissue of these organs at 24 hr postinoculation. These results indicate that both air sacs and lungs can be the portal of entry for E. coli into the systemic circulation, probably via damaged epithelium.