Ovaries from a marsupial, the tammar wallaby (Macropus eugenii), were grafted into a eutherian recipient at known stages of development to ascertain whether normal development would occur. Xenografted ovaries from pouch young < 20 days old, before the onset of meiosis, retained few germ cells and developed tubule-like structures reminiscent of seminiferous cords. Ovaries from 50-day-old pouch young, which contain primordial follicles, developed into antral follicles and corpora lutea within the eutherian host, and produced hormones that stimulated the reproductive tract of the host. The timing of onset of antrum formation and the progress of follicle development were advanced relative to the timing of events in ovaries in situ. Frozen-thawed ovaries from 50-day-old donors developed into preantral follicles, but at a reduced rate and number. This finding shows that gonads of a marsupial species can develop as xenografts in a eutherian, forming large antral follicles. Accelerated follicular development in xenografts provides a potentially valuable model for studying the factors that control follicle development. Assisted reproduction of endangered marsupials may also be feasible using follicles from pouch young grown as xenografts in a eutherian host.