Burbot ( Lota lota maculosa ), also known as freshwater cod, have shown strong potential as a new aquaculture species in the western United States . Previous b urbot research at the Cain Fish Health Lab (University of Idaho ) has found that this fish species is relatively refractory to pathogens present in rainbow trout production systems, such as F. psychrophilum and F. columnare , potentially providing an opportunity for polyculture in existing salmonid production systems. However, it has been found that burbot are susceptible to infection with pathogenic Aeromonas sp , which cause Motile Aeromonas Septicemia (MAS) in many fish species. As such, two recent trials were conducted to assess the virulence of novel Aeromonas sp. isolates, including A. hydrophila . In Trial A, triplicate tanks of 25 fish per tank (mean weight of 40g) were challenged with a known virulent Aeromonas sp. isolate (A141), along with a newly recovered IR034 isolate at approximately 1 x 108 CFU mL-1 in a 100 µl IP-injection. Mock controls were also challenged using culture media ( tryptic soy broth). Endpoint cumulative percent mortality (CPM) at 21 days post-challenge (dpc ) was 49.3± 22.0% for A141 and 4.0±0 .0% for the less virulent IR034. A similar trial was conducted with 55.0 g burbot, incorporating another recent isolate, IR004 (8 x 107 CFU mL-1) , along with a previously reported (in warmwater species) virulent strain of A. hydrophila (ALG-15-097 ; 3 x 107 CFU mL-1). Both isolates were found to be virulent, displaying a CPM of 85.3±6.1% for the IR004 and 97.3±4.6 % for the ALG-15-097 at 21 dpc. Through the initial infection period (0h, 48h and 120 h post-challenge) spleen and kidney samples were collected from three fish per tank for immune gene expression analyses to monitor changes from the onset of infection. Additionally, phagocytic capacity of head kidney-derived leukocytes was assessed using flow cytometry. Trial results indicate that burbot are susceptible an array of different Aeromonas sp. isolates , including A. hydrophila . Based on the challenge outcomes , both A141 and IR004 will be used as candidate isolates to develop a killed vaccine for MAS infections and examine the vaccination response in this unique fish species.