Breeding trials between karyotypically distinct species of Petrogale produced a number of hybrids and backcross hybrids of which some of the females were fertile. The only fertile male hybrid resulted from parents currently considered to be conspecific, but with different karyotypes. To investigate the possibility that hybridizations occur in other genera, we examined three Thylogale species, two of which, T. thetis and T. stigmatica, are sympatric over parts of their range in eastern coastal Australia, while the third, T. billardierii, is now found only in Tasmania. Female hybrids of T. thetis (female) x T. stigmatica (male) were fertile when mated with T. stigmatica, but in male hybrids the testes and seminiferous tubules failed to reach full-blood size and the tubules contained only primary spermatocytes. In contrast, the T. stigmatica (female) x T. thetis (male) crosses failed to produce hybrids. However, the only female backcross hybrid to produce young to date mated with a T. thetis. Female T. billardierii have not produced young with either T. thetis or T. stigmatica males and T. billardierii males could only be caged with conspecifics. Presumably behavioral barriers prevent regular gene flow in the wild, but in unusual conditions of habitat change (such as after fire or forest degradation) occasional gene transfers could be expected.