Two hypersusceptible strains of Pseudomonas aeruginosa, which were inhibited by 0.78 or 1.56μg/ml of ampicillin, were isolated from the sputum of two different patients in 1984. These highly ampicillinsusceptible strains of P. aeruginosa accounted for 0.5% of the 384 strains examined. These strains had high susceptibility for not only ampicillin but also other penicillins and third-generation cephems, though they had not for tetracycline or gentamicin. The beta-lactamase level of P. aeruginosa T-25, a highly ampicillin-susceptible isolate, was increased slightly by growing the cells for 2 hours in the presence of cefoxitin or ampicillin as an inducer. The MICs of ampicillin for P. aeruginosa T-25 and SSP were reduced very slightly when the subinhibitory concentration of ethylenediaminetetraacetic acid (EDTA) was added to the medium. Similar results were obtained by the use of the highly antibiotic-susceptible permeability mutant of P. aeruginosa No. 1771 provided by Prof. Richmond. The potent antibacterial activity of ampicillin against P. aeruginosa T-25 is considered to be due to high permeability of the outer membrane.