Pseudomonas aeruginosa is the most common bacterial isolate obtained from patients with cystic fibrosis of the lungs. Recently, however, new multiresistant organisms have emerged, whose identification may be difficult and whose pathogenic role proves hard to define. Of the 71 strains isolated from 24 patients with cystic fibrosis during acute flareups of pulmonary symptoms, 48 turned out to be Pseudomonas aeruginosa (67.6%); 11 were Pseudomonas non-aeruginosa (15.5%); and 12 were Achromobacter xylosoxidans (16.9%). Each bacterial isolate was tested for sensitivity to nine antibiotics (ceftazidime, azlocillin, piperacillin, aztreonam, cefsulodin, cefoperazone, amikacin, tobramycin, and sisomycin) in terms of minimum inhibitory concentration and minimum bactericidal concentration values. In this series, Achromobacter xylosoxidans proved the species least responsive to treatment, and ceftazidime the most active antibiotic both against Achromobacter and against strains of the genus Pseudomonas. Twenty-three different associations of ceftazidime with aminoglycosides, tested for activity on the multiresistant strains, failed to show synergism of action.