The susceptibility to oxolinic acid of 48 stains of Aeromonas salmonicida was established using the disc diffusion and agar dilution protocols of Alderman and Smith (2001) [Alderman, D., Smith, P., 2001. Development of draft protocols of standard reference methods for antimicrobial agent susceptibility testing of bacteria associated with fish disease. Aquaculture 196, 211-243.]. Both methods generated data that had a bimodal distribution. With respect to the agar dilution method an unambiguous breakpoint MIC of 0.25 mg/l allowed all strains to be classified as susceptible or tolerant. The equivalent figure for the disc diffusion data was 30 mm. Spearman's rank correlation analysis demonstrated a high degree of correlation (r=0.89) between the zone diameters and the log of the MIC values for members of the strain set.The breakpoints estimated in this work are compared to those estimated by other workers from data generated from different protocols. The issue of the clinical relevance of the breakpoints generated from the distribution of susceptibility data is discussed. (C) 2005 Elsevier B.V. All rights reserved.