Isolation frequency of multiple-antibiotic resistant Pseudomonas aeruginosa (MARPA) was 11.9% (fifty six strains) of a total of four-hundred seventy-one strains of P. aeruginosa isolated from clinical specimens at the Kyorin University Hospital from October 1994 to December 1996. Eighteen strains of MARPA and thirteen strains of antibiotic sensitive P. aeruginosa (ASPA) isolated from clinical specimens in internal medicine ward A were determined O serotype, and characterized with production of pyocyanin, pyoverdin, hemolysin, elastase, and caseinase. Sixteen strains (88.9%) of MARPA were identified as serotype C. The ability to produce pyocyanin, hemolysin, elastase, and caseinase was not detected in all MARPA. One side, thirteen strains of ASPA showed various serotypes, i.e., B: 5 strains (38.4%), G: 4 strains (30.8%), C: 2 strains (15.4%), E: 1 strain (7.7%) and unknown type: 1 strain (7.7%), and the production of both hemolysin and pyoverdin was observed in 13 strains (100%), pyocyanin in 8 strains (61.5%), elastase and caseinase in 9 strains (69.2%) of ASPAs, which suggests that ASPAs do maintain the synthetic ability of pathogenic factors and pigments, but MARPAs do not. These results indicate that from epidemiological points of view the current strains of MARPA spread from one clone within the internal medicine ward A with nosocomial outbreak by serotype C.