The rodent Calomys musculinus is the reservoir of the Junin virus, which causes Argentine hemorrhagic fever. A phylogeographic approach was used for the analysis of 6 populations from central Argentina based on cytochrome-b sequences from 27 individuals. Thirteen haplotypes were detected, each closely linked in the phylogenetic network; 2 of the most frequent haplotypes were internal, whereas the majority were unique to single populations and presented distal positions in the network. Most of the variance was explained by within-population genetic diversity. Results of a mismatch distribution analysis and the deviation from neutrality confirmed that C. musculinus has experienced a recent range expansion; genetic drift has apparently been the main force acting at present, with low to moderate current gene flow. These results are in agreement with previous studies based on polymerase chain reaction–restriction fragment length polymorphism data of the mitochondrial D-loop region, and they support the reliability of both types of techniques.