Infectious hematopoietic necrosis virus (IHNV) is a rhabdovirus that has caused disease and mortality in Pacific salmonid fishes throughout their native range. As a result of the sharing of virus isolates between Federal, State and Tribal agencies, a comprehensive understanding of the phylogenetic diversity of IHNV has been developed by researchers at the USGS Western Fisheries Research Center in Seattle, Washington. Sequencing of a 303 nucleotide region within the glycoprotein gene from numerous virus isolates has identified three genetic subgroups of IHNV (U, M, and L) that have distinct geographic ranges. Further investigation has demonstrated that these subgroups exhibit host specific disease and mortality. While genome sequencing has shed a great deal of light on the ecology of IHNV, the transmission mechanisms of IHNV among Pacific salmonid fishes remain unclear. In order to best manage Pacific salmonid fishes throughout their native range, it is critical to understand how IHNV is evolving among salmonid fishes and how current management practices may be inducing rapid virus evolution.