Chromosomal rearrangements in three karyotypically distinct, but morphologically and biochemically similar, races of rock wallabies (Petrogale) from North Queensland. Australia. Were examined by C-and G-banding of cultured fibroblasts and by analysis of synaptonemal complexes from gonads of hybrid individuals. The assimilis race (2n = 20) had a submetacentric 6; 10 centric fusion, whereas the Mt. Claro(2n = 20)and Mareeba (2n = 18) races shared an identical submetacentric 5; 10 centric fusion. The 5 of this 5; 10 fusion was inverted in both races with respect to the free 5 of the assimilis race. Moreover, the Mareeba race had a unique acrocentric 6;9 fusion. Thus, while all three races are closely related, it is apparent from the chromosomal orientation that the Mareeba and Mt. Claro races are more closely related to each other than either is to the assimilis race.