Although viruses have been associated with mortality outbreaks in different mollusc species, little information is available on their exact affiliation and taxonomic position. Herpes viruses have attracted particular attention because of their economic and ecological impact on both farmed and wild marine molluscs over the past 20 years. Investigations based on molecular approaches have been carried out on the herpes virus infecting Pacific oysters in France. Molecular and ultra-structural data were used to classify the virus under the name ostreid herpesvirus 1 (OsHV-1), as the first known species in the family Malacoherpesviridae. Since 2008, massive mortality outbreaks were reported in several Crassostrea gigas oyster farms in France, Ireland, and the United Kingdom. The outbreaks were attributed to adverse environmental conditions associated with a particular variant of OsHV-1, called OsHV-1 mu Var.