Blastocysts in the Hill Collection from Trichosurus vulpecula, Pefrogale penicillata, Macropus ruficollis (= M. rufogriseus), Macropus parma, Onychogalea fraenata, Bettongia gaimardi, Perameles obesula (=lsoodon obesulus), Perameles nasuta, Dasyurus viverrinus, Didelphis aurita (=D. marsupialis) and Didelphis virginiana were examined. They ranged from incomplete unilaminar blastocysts to late bilaminar blastocysts. The mode of formation of the unilaminar blastocyst appeared to be influenced by the presence or absence of the yolk mass. A unilaminar blastocyst lined by uniform protoderm cells occurred in a wide variety of marsupials. Differentiation of the unilaminar blastocyst into embryonic and extra-embryonic areas occurred at different stages of development. In macropodids and Didelphis it was found in small blastocysts soon after blastocyst completion. In dasyurids, Perameles and some other groups it was found in larger blastocysts, at least four cell generations after blastocyst completion. The first histological signs of differentiation of the unilaminar blastocyst into embryonic and extra-embryonic areas varied between different marsupials. In Didelphis, enlarged endoderm mother cells developed from the protoderm cells of one hemisphere. The protoderm cells of this hemisphere later differentiated as embryonic ectoderm and the endoderm mother cells gave rise to the primary endoderm. In D. viverrinus, bandicoots and T. vulpecula, the protoderm cells of one hemisphere differentiated simultaneously into cuboidal embryonic ectoderm and endoderm mother cells. In P. penicillata, M. ruficollis and M. parma the protoderm cells of one hemisphere proliferated to form a multilayered embryonic area which later differentiated into embryonic ectoderm and primary endoderm.