Posterior neuropore (PNP) closure coincides with the end of gastrulation, marking the end of primary neurulation and primary body axis formation. Secondary neurulation and axis formation involve differentiation of the tail bud mesenchyme. Genetic control of the primarysecondary transition is not understood. We report a detailed analysis of gene expression in the caudal region of day 10 mouse embryos during primary neuropore closure. Embryos were collected at the 27-32 somite stage, fixed, processed for whole mount in situ hybridisation, and subsequently sectioned for a more detailed analysis. Genes selected for study include those involved in the key events of gastrulation and neurulation at earlier stages and more cranial levels. Patterns of expression within the tail bud, neural plate, recently closed neural tube, notochord, hindgut, mesoderm, and surface ectoderm are illustrated and described. Specifically, we report continuity of expression of the genes Wnt5a, Wnt5b, Evx1, Fgf8, RARβ₯, Brachyury, and Hoxb1 from primitive streak and node into subpopulations of the tail bud and caudal axial structures. Within the caudal notochord, developing floorplate, and hindgut, HNF3β£, HNF3β€, Shh, and Brachyury expression domains correlate directly with known genetic roles and predicted tissue interdependence during induction and differentiation of these structures. The patterns of expression of Wnt5a, Hoxb1, Brachyury, RARβ₯, and Evx1, together with observations on proliferation, reveal that the caudal mesoderm is organised at a molecular level into distinct domains delineated by longitudinal and transverse borders before histological differentiation. Expression of Wnt5a in the ventral ectodermal ridge supports previous evidence that this structure is involved in epithelial-mesenchymal interaction. These results provide a foundation for understanding the mechanisms facilitating transition from primary to secondary body axis formation, as well as the factors involved in defective spinal neurulation.