In the genus Peromyscus cells of all species contain 48 chromosomes; however, the fundamental number varies from 56 (P. crinitus, P. boylei) to 96 (P. eremicus). In some cases biarmed chromosomes are the result of pericentric inversions, while in others they are the result of addition of large amounts of constitutive heterochromatin. Flow microfluorometric DNA-per-cell determinations demonstrated that in some species (P. eremicus) the genome is increased by 36 % over the amount of DNA found in most mammalian species. Studies of unique karyotypes with increased amounts of DNA added as constitutive heterochromatin may ultimately help in the elucidation of the mechanisms involved in karyotype evolution and speciation.