Fertility, synaptonemal complexes (SCs) and spermiogenesis were examined in hybrids bred from four chromosomally different species of rock wallaby. Each of the four male hybrids had different forms of chromosomal heterozygosity (i.e., a trivalent, a quadrivalent, a pentavalent, and a trivalent plus two centric shifts). Testes and seminiferous tubules of each hybrid were smaller than those of full-bloods, and no hybrid produced young when paired with parous females. The degree of chromosomal heterozygosity in the hybrids was associated with the degrees of asynapsis, XY-autosome associations, and disruption of spermatogenesis. In only one hybrid were all stages of spermiogenesis commonly seen, and in this animal, sperm abnormalities were common. SC formation was consistent with that expected from the G-banded karyotypes, except that in one hybrid the chromosomes involved in the presumed centric shifts did not pair as expected but instead formed heteropycnotic masses. In addition, the studies demonstrated the links between the nonpairing X and Y chromosome and revealed an array of patterns of abnormalities at the ultrastructural level in spermiogenesis of hybrid marsupials.