P.aeruginosa is a microorganism that causes certain nosocomial infections and causes difficulties in the treatment of cystic fibrosis patients. The in vitro susceptibility of 60 P.aeruginosa cystic fibrosis isolates and 59 P.aeruginosa strains isolated from other lower respiratory tract infections, against several antibiotics, were evaluated by microdilution technique as recommended by NCCLS. MIC50 and MIC90 values were determined for each antibiotic tested. It was found that meropenem (50%), amikacin (50%), cefepim (46%) and imipenem (41%) were the most effective antibiotics against the intensive care unit isolates. The isolates from cystic fibrosis patients and outpatients were found to be more susceptible than the nosocomial strains, against most of the antibiotics tested.