Mucoid strains of Pseudomonas aeruginosa isolated from the sputa of patients with cystic fibrosis or in vitro through selection with phage were more resistant to carbenicillin, flucloxacillin and tobramycin and more sensitive to tetracycline than related non-mucoid strains. The observations led to the development of a technique for the isolation of mucoid variants in vitro based on enhanced resistance to carbenicillin. Mucoid variants were isolated from various strains of Ps. aeruginosa and the exopolysaccharide found to be similar to that obtained from mucoid Ps. aeruginosa isolated from patients with cystic fibrosis.