This study demonstrates that 3 well characterized laboratory strains and 6 clinical isolates of P. aeruginosa (3 mucoid and 3 non-mucoid) inhibit the VX-809 and VX-809 + VX-770 stimulated increase in F508del-CFTR Cl secretion in a cell line (CFBE) and in primary cultures of CF-HBE cells. This evidence concerns the gene CFTR and cystic fibrosis.