We estimated the minimum inhibitory concentrations of following twenty-one antimicrobial agents against 161 strains of Pseudomonas aeruginosa isolated from clinical specimens during 1979. Antimicrobial agents were 5 penicillins (carbenicillin, sulbenicillin, piperacillin, apalcillin and ticarcillin), 3 cephalosporins (cefsulodin, cefoperazone and ceftizoxime), 5 aminoglycosides (gentamicin, tobramycin, dibekacin, amikacin and kanamycin), 3 tetracycclines (tetracyline, doxycycline and minocycline), chloramphenicol, colistin, polymyxin B, nalidixic acid and pipemidic acid. Among penicillins piperacillin and apalcillin showed strongest antibacterial activity and the activity of carbenicillin was weakest. Highly resistant strains against carbenicillin increased rapidly until 1975. Among cephalosporins cefsulodin showed strongest antibacterial activity and its activity was stronger than piperacillin. Antibacterial activity of cefoperazone and ceftizoxime were weaker than cefsulodin, but stronger than carbenicillin and sulbenicillin. Among aminoglycosides kanamycin showed very weak antibacterial activity, but another four drugs showed very strong activity. But resistant strains against these four drugs increased gradually and over 20% of strains was resistant to gentamicin. Among gentamicin, tobramycin and dibekacin cross-resistance was observed, but not between amikacin and these three drugs. Recently the number of strains resistant to both amikacin and gentamicin increased. Tetracyclines, chloramphenicol and nalidixic acid showed rather weak activity, but colistin and polymyxin B showed very strong activity and resistant strains against these drugs were very few. Pipemidic acid showed rather stronger activity than nalidixic acid, but its activity was weaker than aminoglycosides excluding kanamycin.