This paper examines aspcets of the resistance of Aeromonas salmonicida isolates to the major fish farming antimicrobials. The antimicrobial resistance of an extensive chronological series of isolates appears to reflect the changing use of different compounds in tish farming since 1969. Repetitive challenge by 5(1% MIC concentrations of 4-quinolonc antimicrobials against selective isolation of viable colonies resulted in a 7 2 x increase in resistance in 12 generations, demonstrating the likely outcome of drug misuse.