The activities of five fluorinated 4-quinolones, namely, sarafloxacin, enrofloxacin, PD 127391, PD117596, and CI934, against the fish pathogen Aeromonas salmonicida were investigated and compared with that of oxolinic acid. The results indicated that with the exception of CI934, these drugs are more active than oxolinic acid in terms of MIC. No inoculum effect was observed, but the drugs were less active at 10°C. The presence of 3% NaCI caused an increase in drug activity. Resistance to the drugs appeared to be fairly stable, with only a small decrease in activity after 10 successive passages of the test strains on drug-free tryptone soya agar.