The arenaviruses are primarily viruses of rodents, although a minority of them cause serious disease in humans. They were originally grouped together on the basis of their appearance in the electron microscope (Dalton et al.,1968; Murphy and Whitfield, 1975) and serological cross-reactivity in complement fixation and immunofluorescence tests (Rowe et al.,1970; Casals et al., 1975; Wulff et al., 1978). The production and use of monoclonal antibodies have reinforced and refined our ability to group these viruses and to recognize variation among strains of the same virus (Buchmeier et al., 1981; Howard et al., 1985; Sanchez et al., 1989; Ruo et al., 1991) Analysis of the structural components of several members of the family has confirmed their general similarity at the molecular level (see Bishop and Auperin, 1987; Buchmeier and Parekh, 1987, for reviews) and has provided, through nucleotide sequencing, a means by which we can begin to make quantitative estimates of the relationships among these viruses. Here I have collected together a directory of the arenavirus sequence data currently available in widely distributed databases, multiple alignments of nucleocapsid (N) and glycoprotein precursor (GPC) protein amino acid sequences, and present phylogenetic trees based on these sequences, which graphically summarize the relationships among these viruses.