The relative prevalence of mucoid strains compared with non-mucoid strains of Pseudomonas aeruginosa has been investigated in all routine bacteriological specimens received in a department of clinical microbiology during 1973. Pseudomonas aeruginosa was isolated from 1054 of the specimens (5.7 per cent) representing 53 patients with cystic fibrosis (551 isolates) and 169 patients with other diseases (503 isolates). The relative prevalence of mucoid strains was significantly higher in specimens from patients with cystic fibrosis (80 per cent) than in specimens from patients with other diseases (3 per cent). Considering patients without cystic fibrosis, the relative prevalence of mucoid strains was low in specimens from all anatomical regions, and no special preference of these strains for the respiratory tract could be demonstrated in these patients, in contrast to the situation in patients with cystic fibrosis. Alternation between mucoid strains and non-mucoid strains in subsequent specimens was observed in 33 of the patients.