Survival of Aeromonas salmonicida was monitored during prolonged incubation in either distilled water or lake water. Culturability was determined from colony forming units enumerated on tryptone soy agar, whilst flow cytometry was used for direct analysis of viable cells after staining with fluorescent dyes which differentially stained bacteria in relation to defined cellular properties. Over time, populations of culturable cells steadily declined and were not detected after 10 days incubation in distilled water or 33 days in lake water. Flow cytometric analysis revealed that cellular properties related to viability were lost shortly after culturable cells became undetectable in distilled water. In contrast, those incubated in lake water showed little change in these properties over a 57-day experimental period. The implications of these differences are discussed, and it is concluded that A. salmonicida is capable of remaining intact and active upon prolonged incubation in lake water, although this does not conclusively prove viability.