In salmonid production, the most common pathogenic threats to fish stocks arise from members of the family Flavobacteriaceae, most notably: Flavobacterium psychrophilum, F. branchiophilum, and F. columnare. Clinical cases of disease caused by new and emerging bacterial strains in this family (Flavobacteriaceae) have been increasing. In this study, a n assessment of virulence was conducted on four novel Flavobacteri um spp. isolated from clinical disease cases from hatcheries in the Western region of the United States: 11299 (WA), 3gHT (UT), CA143 (CA), and CO45A (CO). Juvenile rainbow trout, O. mykiss , were challenged with the isolates mentioned above. Each isolate was administered via intramuscular injection at a dose of 10 7 CFU∙fish-1 . Results showed that these four Flavobacteriaceae isolates were highly virulent and caused acute clinical disease. Cumulative percent mortality (CPM) for isolates 11299, 3gHT, CA143 , and CO45A was 46.7%, 93. 3%, 96.7%, and 96.7%, respectively . Following virulence analysis, an investigation of cross-protection against these four isolates was initiated. A live-attenuated F. psychrophilum vaccine, previously developed and demonstrated to provide protection against an array of F. psychrophilum strains as well as mixed groups of Chryseobacterium spp. and Flavobacterium spp., was utilized. Juvenile rainbow trout (Oncorhynchus mykiss) were vaccinated by immersion and high serum anti-F. psychrophilum antibody titers developed. Fish were then challenged with isolates 11299, 3gHT, CA143, CO45A, and a mixed combination. Cross-protection to isolate 11299 was demonstrated with a relative percent survival (RPS) of 36.21% compared to the unvaccinated 1 1299 control group (p< 0.05). Cross-protection was also observed in groups challenged with the mixture of the four isolates with an RPS of 41.46% (p<0.05). However, no protection was observed in groups challenged with the 3gHT, CA143, and CO45A isolates alone. Results from this s tudy confirm that these emerging flavobacterial strains cause clinical disease and suggest that an attenuated F. psychrophilum vaccine may provide some benefit through cross-protective immunity.