Rock wallabies, Petrogale, exhibit chromosome diversity that is exceptional in marsupials, with 20 distinct chromosome races being recognized. Many of the karyotypic changes identified within Petrogale appear to be recent, although the rate of chromosome evolution varies between taxa. While the patchy distribution of Petrogale and their social structure would facilitate the fixation of novel rearrangements, these factors alone do not explain the pattern of chromosome evolution shown in this group. The chromosome changes that have come to characterize each taxon may offer selective advantages in the particular areas occupied, or it may be that these rearrangements play an important role in reproductive isolation. In Petrogale, the taxa with the largest number of chromosome rearrangements are those that are sympatric, or have multiple zones of parapatry, with other members of the genus. Male hybrids from a variety of chromosomal admixtures were found to be sterile, but with those heterozygous for the least complex rearrangements being least affected. As expected, equivalent female hybrids were less severely affected. Chromosomal and genic changes both appear important in these processes.