The extra-embryonic yolk sac is the first site of haematopoiesis in the mouse conceptus. This diaphanous membrane encapsulates the developing embryo proper and consists of an inner mesoderm-derived layer of blood vessels and haematopoietic progenitors, and an outer visceral endoderm-derived layer of absorptive epithelial cells. To better characterise the developmental processes regulating haematopoiesis, we performed flow cytometric analyses of epithelial and haematopoietic surface marker expression. Epcam+ CD45- visceral endodermal cells were identified and when isolated by FACS showed a epithelial morphology. A distinct population simultaneously expressing Epcam and CD45 was also present. This population was present only during the window between E10.5-14.5, co-expressed E-cadherin as well as the macrophage markers F4/80, CD11b, CD206 (Mannose receptor) while lacking CD115 (M-CSF receptor), VCAM1, CD169 and CD207 (Langerin) expression. These cells were observed on the outer surface of the visceral endoderm layer. Hence, we have named these cells gefyrocytes, from the greek word gefyros meaning bridge. Gefyrocytes are derived from the conceptus not the mother and are dependent upon Runx1 and c-Myb but not Evi-1 expression. FACS-purified gefyrocytes possessed a classical macrophage morphology. These findings strongly indicate that gefyrocytes are a newly identified yolk sac-derived macrophage population which, due to their unique location, may serve in fetomaternal immune tolerance or function, or yolk sac remodeling.