Aeromonas salmonicida, the causative agent of furunculosis in fish, has been shown to become non-culturable but viable after inoculation in fresh water. The onset of non-culturability is entirely predictable, but can be delayed by inoculation at high concentration or amendment with nutrients. This paper reports that non-culturability can be postponed by the addition of both the amino acids methionine and arginine to microcosms inoculated with A. salmonicida. During these experiments, A salmonicida decreased in cell size and became rounded. This was regardless of whether it received an amino acid supplement or not. We observed that cells receiving both amino acids remained culturable despite their reduction in cell size to less than 1 μm. Therefore, it would appear that the reduction in size and associated morphological change cannot be taken as an indicator of non-culturability, although it may be a significant step in that direction in some cases.