Vaccination of fish against many different pathogenic organisms has made it possible to rear Atlantic salmon in net pen cages and produce fish commercially around the world. In fact, vaccine use is critical for the continued growth of the aquaculture industry and researchers are continually looking to develop new and improved vaccines for a wide variety of fish pathogens. Fish vaccines have been formulated from killed or attenuated pathogens, recombinant viral proteins or peptides, and most recently, plasmid DNA encoding viral proteins (1 –7 ). The use of DNA vaccines for the control of viral diseases of fish is particularly appealing since this type of vaccine eliminates the need to purify the viral pathogen or immunoprotective antigen. Other advantages are the elimination of any possibility of reversion to virulence since the DNA vaccine encodes only a portion of the viral genome, the relative stability of the DNA preparation; the ease of DNA preparation; and, importantly, the elicitation of a robust immune response in fish.