Six hundred and fifty hospital isolates of Pseudomonas aeruginosa from Australian sources have been examined for high-level resistance to a number of antibiotics. Fifty-four strains were resistant to one or more of the antibiotics, and four of these strains carried as R-plasmid conferring resistance to streptomycin, tetracycline and sulphanilamide, and belonging to incompatibility group P-2. Possible reasons for the low incidence of R-plasmids in P. aeruginosa from Australian sources are discussed.