Four non‐pigment‐producing isolates and two pigment‐producing isolates of Aeromonas salmonicida sp. salmonicida were isolated from the head‐kidney of diseased farmed Atlantic salmon, Salmo salar L. The cultural, morphological and biochemical features of the isolates were compared with those of reference strains. Injection and cohabitation experiments were performed. The only difference between the non‐pigment‐producing isolates and the pigment producing reference strains of A. salmonicida ssp. salmonicida was the inability of the former to produce pigment. In the injection experiments, the investigated non‐pigment‐producing isolate produced a significantly higher mortality compared with the mortality caused by the reference strain, whereas no difference in mortality was detected in the cohabitation experiments.