Aeromonas salmonicida is the causal agent of furunculosis in various fish species. Detection methods include culturing, serology and molecular biology techniques. Controversy surrounds its possible independent existence in water; enzyme-linked immunosorbent assay and the polymerase chain reaction have detected A. salmonicida in the absence of colony-forming units, but cells that are nonculturable may be significant to fish pathology. Furunculosis is probably transmitted by the pathogen's entry into gills, mouth, anus and/or surface injury of fish through contact with infected fish or contaminated water. Disease-control is possible by good husbandry practices, disease-resistant stock, improved diets, nonspecific immunostimulants, antimicrobial compounds and vaccines.