The sequence of the nucleocapsid (N) protein of Machupo virus (causative agent of Bolivian haemorrhagic fever) has been determined, and used to infer a phylogenetic relationship to other arenaviruses. The relationship of the virus to Junín and Tacaribe viruses, together with previous demonstrations of antigenic similarity and cross-protection by heterologous viruses, suggest that vaccines developed against Argentine haemorrhagic fever might also be effective against the Bolivian disease.