Abstract
Chromosomes were analyzed from 38 hispid cotton rats, currently assigned to the species Sigmodon hispidus, from populations in southeastern and western United States. Cotton rats from southeastern United States had a 2N of 52 and an F. N. which varied from 52 to 54. Specimens from Obion County, Tennessee, and Highlands County, Florida, were found to be polymorphic with a varying number of arms on the largest pair of autosomes. Cotton rats from Arizona had a 2N of 22 and an F. N. of 38; each pair of chromosomes is distinguishable, and a numbering system is proposed. The cytological data suggest that cotton rats from the southeastern populations and those from the Arizona populations belong to separate species, though morphological characters do not indicate such a difference.
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