The majority (86.6%) of patients with cystic fibrosis were found to be carriers of Pseudomonas aeruginosa. None of them, however, carried P. aeruginosa in their nares. In contrast, none of the non-CF family members of the patients with CF were carriers of P. aeruginosa. For example, only 4 of 468 cultures from skin, throat, and nares of the family members were positive for P. aeruginosa. Isolations of P. aeruginosa from the same CF patients were often of the same pyocine type. No specific pyocine type of P. aeruginosa was predominant in patients with CF. Isolations of P. aeruginosa from siblings with CF may or may not be of the same pyocine type as that of the family proband. Colonization of a patient with CF by P. aeruginosa is not a threat to the non-CF members of the family.