The purpose of this study was to define the taxonomic relationship of an arenavirus principally associated with the southern plains woodrat (Neotoma micropus) in southern Texas to other New World arenaviruses. The results of independent analyses of glycoprotein precursor amino acid sequences and nucleocapsid protein amino acid sequences indicated that the arenavirus in southern Texas is novel (proposed species name Catarina virus) and phylogenetically most closely related to Whitewater Arroyo virus, which is principally associated with the white-throated woodrat (Neotoma albigula) in northwestern New Mexico. Together, the close phylogenetic relationship between Catarina virus and Whitewater Arroyo virus and the association of these viral species with congeneric rodent species support the notion that the principal host relationships of some New World arenaviruses are a product of a long-term shared evolutionary relationship between the virus family Arenaviridae and the rodent family Cricetidae.