The main objective of this project is to obtain knowledge on the genetic variation of IPNV and risk factors for IPN in Norway with emphasis on Atlantic salmon. Information about the prevalence of different virus strains is important for the fish farmers , fish health services, fish health authorities and vaccine companies to combat and control the disease The molecular biology of IPNV is well known, but a genetic characterisation of the virus causing disease outbreaks and of virus from carrier fish has n ot been performed systematically. Tissue samples collected from sea farms experiencing clinical IPN will be examined for the presence of IPNV, and in the case of detection of virus, the genes encoding viral proteins with documented strain variations (Segm ent A) will be sequenced. Information on virus variants will be associated with factors on management, environment and fish status in risk factor analyses. To reveal any virus introductory link between hatchery and sea sites, A. salmon in selected areas a nd farms will be followed from the parr stage in hatcheries, through transfer to sea and for four months post sea transfer with regard to the presence of genetic variants of IPNV and promoting components for disease outbreaks.