Strains of Aeromonas salmonicida ssp. salmonisalrnonicida ssp. masoucida, and several isolates not cida lacking pigment production were isolated from brown assigned to any of the valid subspecies, trout Salmo trutta m. lacustris and sea trout S. trutta m. trutta Recently a fourth subspecies, A. salmonicida ssp. cultivated in fresh water in south Finland. The bacteria isolated showed only minor deviations in biochen~ical character-

The biochemical properties of 105 Aeromonas salmonicida (AS) strains were examined in order to find criteria for distinguishing between 'typicals' and 'atypicals' and for further subdividing the 'atypicals'. One hundred of the strains had been isolated from fish at 35 fish farms and 5 from wild fish between 1986 and 1991. The fishes involved were as follows: lamprey, whitefish, rainbow trout. salmon, sea trout, brown trout, arctic char, lake trout, grayling, dace and roach. Typical (AS subsp. salmonicida) and atypical AS strains could be differentiated using 10 biochemical tests: production of acid from saccharose, salicin, a-methyl-D-glucoside, L-arabinose and arbutin, production of gas from glucose and maltose, hydrolysis of aesculin, haemolysis, and hydrolysis of Tween 80. Biochemically, the atypical isolates could not be classified as any of the proposed subspecies and could not be clearly subdivided. All of the AS strains were sensitive to oxolinic acid, trimethoprim-sulpha, chloramphenicol and nitrofurantoin. Oxytetracycline-resistant typical strains were isolated from 9 of the farms.