When applied to the psychrophilic and psychrotropic bacteria associated with diseases of fish, the disc‐diffusion method generally fails to provide correct estimates of minimal inhibitory concentrations (MIC). A microdilution method using a medium containing agar (15 g/l) and 10 selected drugs has been developed. Serial four‐fold dilutions of the drugs were performed in specially designed dilution plates, which were prepared and stored at −30° or −70°C. Although its reliability sometimes appeared to be influenced by the temperature at the time of testing, the temperature and duration of freezing and the nature of the drugs, the microtest provided results as accurate as other reference methods. In repeated experiments very rare major discrepancies were noted and minor variations of one dilution step were below 10, except with tetracyclines and the sulphonamide‐trimethoprim mixture. The advantages of the method and the optimum conditions for use in fish disease diagnosis ar
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