Examination of 154 specimens representing nine species of the genus Neotoma revealed diploid numbers of 38, 52, 54 and 56. Six species had the diploid number of 52. Extensive polymorphism was found in two species, N. lepida and N. micropus. Chromosomal variation in the subgenus Neotoma was primarily in Fundamental Number and not in diploid number.

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Examination of allozymic variation in a chromosomally highly polymorphic population of Oryzomys documented that the sample consisted of two species, one that was chromosomally monomorphic and the other containing nine centric fusion polymorphisms. The existence of allozymic variation indicates that it is highly improbable that the polymorphic sample was the result of hybridization. These centric fusion polymorphisms appeared to cause little or no negative selection.