Three closely related isolates belonging to the A��� serotype of infectious pancreatic necrosis virus (IPNV) were selected for comparison, to provide insight into the nature of variation in the virulence of IPN viruses. Brook trout fry (Salvelinus fontinalis) were experimentally infected with the three isolates by immersion. Cumulative mortalities over a 62 day period for the three isolates were 67%, 78%, and 93%. The negative control was 3%. Virus titers from whole fish homogenates sampled at peak mortality for each isolate were statistically similar, indicating that quantity of virus does not account for virulence differences. For the two least virulent isolates, the virus titer was inversely correlated with fish weight, whereas for the most virulent isolate, no correlation was observed. Amino acid sequences of the viral capsid protein VP2 were determined using the reverse transcriptase polymerase chain reaction (RT-PCR). There were two amino acid changes at residue 217 and 288 between the two least virulent isolates and the most virulent isolate. These changes might provide a specific molecular basis for the variations in virulence among isolates. The progression of IPN virus infection in the experimentally infected fry was followed using histopathology, in situ cDNA hybridization, and alkaline phosphatase immunohistochemistry (APIH). While microscopic lesions were limited almost exclusively to necrosis of the pyloric caeca and pancreas, positive reactions with in situ hybridization and APIH were observed in tissues throughout infected fish. An IPNV infection appeared to be established in the fish by two routes: by entering the skin/lateral line and diffusing through the muscle, and from the oral region into the gastrointestinal tract by ingestion. In a second experiment, within a group of experimentally infected brook trout fry, external and behavioral signs of IPN disease in moribund fish disappeared, with the fish becoming healthy in appearance. Several of these fish were sampled, along with dead, moribund, and asymptomatic fish (never showed signs of IPN disease). Very few differences were observed among the fish sampled, using histopathology and in situ hybridization. Fish that appeared to recover after displaying signs of IPN disease died within a 2 week period. / Graduation date: 1997