Viral erythrocytic necrosis (VEN) has been associated with large epizootics and high mortality in Pacific herring, Clupea harengus pallasi , from 2 different locations in south-eastern Alaska. Clinical signs included whirling, pale gills, watery colourless blood, discoloured livers, pathognomonic magenta erythrocytic inclusion bodies and histopathological changes consistent with other infectious haemolytic anaemias in higher animals. Transmission electron microscopy confirmed the presence of iridovirus-type particles associated with the cytoplasmic erythrocytic inclusion bodies. Other apparently healthy herring from 4 additional areas in south-east Alaska also had clinical signs of VEN suggesting a wide distribution of the virus in Alaskan Pacific herring populations. Osmoregulatory stress may have precipitated mortalities in fish having severe anaemia caused by the VEN virus. This is the first reported occurrence of VEN in Alaska and the first natural epizootic known to be associated with the disease.