An unidentified myxosporean parasite in the genus parvicapsula has caussed significant mortality in farmed Atlantic salmon in northern Norway. This parasite has never previously been seen in Norway. It is feard that the parasite may also have negative con sequences for wild populations of Atlantic salmon in northern rivers. The parasite shows some similarities to Parvicapsula species, but there are differences that make it probable that the Norwegian parasite represents an unknown species. The source of in fection is not known, and the pathogenicity of the parasite is not understood. The proposed project seeks to describe the parasite, understand how and why the fish dies, find the natural hosts of the parasite, and elucidate its life cycle. Such knowledge will enable The Norwegian Animal Health Authorities to recommend which precautions should be taken to prevent future losses due to the parasite, and answer if Parvicapsula sp. represent any threat to wild Atlantic salmon in nothern Norway. The project wi ll be carried out in close cooperation with the strategic institute programme "Diagnosis of parasitic diseases and zoonoses in animals and fish, utilizing polymerase chain reaction and conventional methos" at NVI, financed by the Research Council (project no. 146566).