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Abstract Background and Objective: Cystic fibrosis (CF) is an autosomal recessive genetic disease and Pseudomonas aeruginosa is one of the most common bacteria colonized in CF patients. Growing resistance of this bacterium to antibiotics now a day is a challenge of controlling infection in CF patient. In this study colonization of CF patients with Pseudomonas aeruginosa and antibiotic susceptibility pattern of isolated strains were examined. Material and Methods: From 100 CF patients, during a year, sputum and bronchial swabs were collected. After culturing the samples, some of them were reported as Pseudomonas aeruginosa using biochemical tests. Mucoid strains of Pseudomonas aeruginosa were identified the same as non-producing alginate strains while for catching single pure colony, repeated passage was used. For determining antibiotic resistance of Pseudomonas aeruginosa to some antimicrobial agents Kirby-Bauer method based on CLSI was used. Results: Of 100 samples, 40 (40%) were positive for Pseudompnas aeruginosa. The prevalence of P. aeruginosa was 23.8, 36.84 and 80% at the age of 1-3, 4-12 and 13, respectively. Conclusion: Statistically, there is a significant difference between age and contracting with Pseudomonas aeruginosa in that the higher the age the more colonization with Pseudomonas aeruginosa. Key words: Pseudomonas Aeruginosa, Cystic Fibrosis, Drug Resistance