Primordial germ cells (PGCs) of the tammar wallaby Macropus eugenii have a distinctive morphology and stain positively for alkaline phosphatase. PGCs are identifiable in embryos with 12 somites, on about day 17 of the 26.5 day gestation period, when they are located in all three germ layers of the developing embryo and in the endoderm of the bilaminar and vascular (trilaminar) yolk sac membranes. PGCs are positive for alkaline phosphatase (ALP) at least between days 17 and 22 of pregnancy. In whole mounts on day 17, three groups of cells positive for ALP occur: about 40 just caudal to the neural tube, and about 20 distributed on either side of the last three somites. By day 21, there are about 150 PGCs in the newly formed gonadal ridges and 275 in the mesenteries. On days 21-22, there are PGCs in the umbilical mesoderm, the dorsal mesentery and the coelomic angles between the dorsal mesentery and the mesonephroi. On day 22, most ALP-positive PGCs are located in the dorsal mesentery, where they occur in groups. They apparently do not migrate through the hindgut endoderm, but occasional PGCs are seen in sites such as the mesonephros, the adrenals, the blood vessels of the yolk sac and in the vicinity of the dorsal aorta and dorsal nerve cord. Between day 23 and day 25, 1 day before birth, most of the 3200-4000 PGCs complete their migration to the gonadal ridges. Although there are marked differences between embryogenesis of tammars and mice, development and the pattern of migration of PGCs in this marsupial mammal are similar to that of eutherian mammals.