In 1991, a matched case-control study was performed in Norwegian freshwater hatcheries on risk factors for infection with Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis. The study was based on replies to a questionnaire mailed to smolt producers, and included 30 infected and 66 non-infected hatcheries, matched by county. The study revealed that the main risk factors for infection with A. salmonicida subsp. salmonicida in freshwater hatcheries were: (1) migration of anadromous fish into the freshwater supply of the hatchery, (2) sharing of personnel with other fish farms, and (3) a high concentration of fish farms infected with A. salmonicida subsp. salmonicida near the hatchery. Results indicate that the high prevalence of furunculosis in Norwegian seawater farms has great impact on the risk of infection with A. salmonicida subsp. salmonicida in hatcheries, and also that the bacterium may be transmitted between fish farms by humans.