An outbreak of gentamicin- and tobramycin-resistant Pseudomonas aeruginosa infection occurred in a surgical ward over a three-month period. Resistant Ps. aeruginosa strains with the same serological, phage, and pyocin type were cultured from the urine of six patients. Identical organisms were found on urine bottles, bedpans, and the hands of attendant staff. Inadequate disinfection played a major role in cross-infection. Isolates of the epidemic strain from each of the patients and of an unrelated but similarly resistant Ps. aeruginosa from one of them could transfer resistance to a recipient strain of Ps. aeruginosa. Resistance to gentamicin, kanamycin, tobramycin, sulphonamides, and mercuric chloride was determined by R factors belonging to Pseudomonas incompatibility group P-3. Aminoglycoside resistance was due to acetylation.