Membrane vesicles were isolated from one beta-lactam-sensitive and three beta-lactam-resistant Pseudomonas aeruginosa clinical isolates from patients with cystic fibrosis. The presence of the chromosomally encoded beta-lactamase in the membrane vesicles was shown by electron microscopy and enzymatic studies. This is the first report of extracellular secretion of beta-lactamase in P. aeruginosa and it seems that the enzyme is packaged into membrane vesicles.