ABSTRACT: A paramyxovirus has been isolated from Atlantic salmon Salmo salar suffering from epitheliocystis. This virus does not cause any mortality when used to challenge disease-free salmon, but has been associated with 2 cases of mortality in salmon farms in Norway. Atlantic salmon paramyxovirus (ASPV) has been suggested as a name for the virus. The ASP virus is a slow-growing virus in cell cultures (rainbow trout gill cells: RTgill-W1). Little is known about its importance and its phylogenetic position is uncertain. Hence, the need for a fast and sensitive diagnostic method for studying the prevalence of this virus in salmon farms and for more basic knowledge about its identity were the motivation for this study. A partial nucleotide sequence (816 bp) from the large protein (L protein) gene of the ASP virus has been sequenced from 2 different isolates. The putative amino acid sequence has been compared with the L protein of other paramyxoviruses. This sequence gives strong support to a relationship between the ASP virus and members of the subfamily Paramyxovirinae, genus Respirovirus.