Chronic colonization and infection of the lung with Pseudomonas aeruginosa is the major cause of morbidity and mortality in cystic fibrosis (CF) patients. We found that polarized CF bronchial and pancreatic epithelia bound P. aeruginosa in a reversible and dose-dependent manner. There was significantly greater binding to CF bronchial and pancreatic cells than to their matched pairs rescued with the wild-type CF transmembrane conductance regulator. Bound P. aeruginosa were easily displaced by unlabeled P. aeruginosa but not by Escherichia coli, an organism that does not cause significant pulmonary disease in CF. In contrast, Staphylococcus aureus, a frequent pathogen in CF, could effectively displace bound P. aeruginosa from its receptor. We found undersialylation of apical proteins and a higher concentration of asialoganglioside 1 (aGM1) in apical membranes of CF compared with rescued epithelia. Incubation of P. aeruginosa with aGM1 reduced its binding, as did treatment of the epithelia with the tetrasaccharide moiety of this ganglioside (Gal beta 1-3GalNAc beta 1-4Gal beta 1-4Glc). Finally, an antibody to aGM1 effectively displaced P. aeruginosa from its binding site and blocked binding of S. aureus to CF cells but not to rescued cells. These results show that the tetrasaccharide of aGM1 is a receptor for P. aeruginosa and S. aureus and that its increased abundance in the apical membrane of CF epithelia makes it a likely contributor to the pathogenesis of bacterial infections in the CF lung.