It has been known for many years that furunculosis affects a wide variety of freshwater fish. A series of experiments is described which involved infection of brown trout and certain non-salmonid species with strains of Aeromonas salmonicida , and the histopathology resulting from these experiments is compared with the histopathology of natural infections of A. salmonicida in both salmonids and non-salmonids. The results described show that the sub-acute form of furunculosis can be reproduced by intra-dermal injections of A. salmonicida ; Klontz et al. (1966) demonstrated this in salmonids; in this study other species were also shown to be susceptible. It was possible to transmit A. salmonicida by cohabiting healthy and diseased fish, although major pathological changes were not observed. However, when wild fish mortalities were investigated, the bacterium was isolated only occasionally, and then only after subjecting the fish to stress, even though clinical and histopathological signs of furunculosis or carp erythrodermatitis were present.