Chromosomal homologies between Neotomodon alstoni, Baiomys taylori and species of Peromyscus for which G-band chromosomal data are available in the literature were identified using G- and C-banding techniques. The primitive karyotype for the peromyscine genera is hypothesized to have a 2n = 48 composed of two or three pairs of biarmed and 20 to 21 pairs of acrocentric autosomes, with heterochromatin restricted to the centromeric regions. Patterns of chromosomal change are presented in the form of a cladogram. The G-banded karyotype of Baiomys taylori is composed of all acrocentric autosomes and is most like that of Peromyscus crinitus, differing only by pericentric inversions in pairs 1, 22, and 23. Neotomodon alstoni shares synapomorphic inversions with several species of Peromyscus. Based on these data, we believe Neotomodon is best considered a congener of Peromyscus.