The survival of Aeromonas salmonicida subsp. salmonicida was investigated in lake water. During a 21-day study A. salmonicida became non-culturable in sterile lake water held at 10 degrees C. The incubation of replicate samples between 5 degrees C and 25 degrees C produced similar results. The recovery of colony-forming units of A. salmonicida from different lake water systems indicated that they survived longer in water that was naturally enriched (eutrophic) or enriched with tryptone soya broth. Flow cytometry, fluorescence light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated that non-culturable cells were present. These cells could not be revived in dilutions of tryptone soya broth, whole dead fish or dissected fish tissue. Although viability could not be proven, it was shown that the morphological integrity found in viable cells was also maintained in non-culturable cells.