Genetic evidence of a novel arenavirus species related to but distinct from lymphocytic choriomeningitis virus (LCMV) was obtained from a rodent belonging to an endemic African subgenus of Mus (Nannomys). The phylogenetic position among Old World arenaviruses of this new arenavirus, named Kodoko virus, was reconstructed based on L and NP genes sequences. The finding of an Old World arenavirus related to LCMV outside the known geographical range of LCMV in a novel rodent species reveals new insights about the evolutionary history of arenaviruses.