Several virulence factors have been reported for motile aeromonads and shown to play a role in pathogenicity. Passage through a host in its corresponding ambient temperature may affect the type and/or amount of virulence factor produced by a particular strain of motile aeromonad. In this study, passage of motile aeromonads through brook trout maintained at 11°C and 22°C did alter the virulence factor profile of some strains. DNase activity of the isolates increased after passage through fish maintained at 11°C, but was not significantly altered after passage through fish maintained at 22°C. Detection of S-layer protein production was reduced by passing isolates through fish. Hemolysin and protease production was altered in isolates after passage through brook trout, but clear trends were not apparent.