The purpose of this study was to characterize 42 paired mucoid and non-mucoid Danish cystic fibrosis (CF) Pseudomonas aeruginosa isolates collected in 1997, by RiboPrinting, antibiotic susceptibility and beta-lactamase activity. Eight P. aeruginosa isolates collected before 1991 were included for comparison. Eighteen of the 42 paired mucoid and non-mucoid isolates showed the same ribotype; the remaining 24 belonged to different ribogroups. Mucoid isolates showed higher susceptibility to antibiotics and lower beta-lactamase activity compared with non-mucoid isolates. Significant differences (P < or = 0.01) between mucoid and non-mucoid isolates were found for the meropenem and colistin MICs for the isolates with the same ribotype, and for the MICs of ceftazidime, piperacillin, aztreonam, meropenem, tobramycin, ciprofloxacin and in the basal levels of beta-lactamase for the paired isolates belonging to different ribogroups. A dominant ribotype 73-S2 with hyperinducible beta-lactamase production and significantly higher MICs of piperacillin, meropenem and tobramycin compared with the other major ribotypes (73-S1, 207-S3 and 227-S8) was present among the 84 CF isolates. The isolates collected before 1991 had an antibiotic susceptibility pattern similar to the 1997 isolates. Despite prolonged and intensive antibiotic treatment, susceptible mucoid isolates were isolated from the CF sputum, possibly because these bacteria are protected from the selective pressure of antibiotics by the resistant non-mucoid isolates co-existing in the biofilm in the lungs of CF patients.