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Karyotypic evolution in the Octodontidae and Abrocomidae is examined by use of C-bands. Karyotypic pathways illustrate chromosomal rearrangements that are more varied than previously thought. A superimposition of tandem fusions and non-Robertsonian mechanisms involving euchromatic segments accounts for the pattern of variation observed. Heterochromatic short arms, although present, do not explain the general biarmed karyotypic condition observed across the genera. Evolution of heterochromatin has proceeded through whole-arm additions and euchromatin transformation, and has been independent of the bidirectional changes in diploid number. The magnitude of the karyotypic differences within the Octodontidae reflects its extensive adaptive radiation in the past, although the family is now represented by relatively few species.