Mammogenesis in rabbit, rat, and human begins with the formation of an elevated ectodermal ridge in the embryo. Attempts to demonstrate a morphologically or histologically equivalent mammary line in the mouse have yielded controversial results. We show here that a mammary line exists in the mouse embryo at embryonic day (E) 11.25 as a concise line of Wnt10b expression and a broader band of Wnt6 expression in the surface ectoderm, between the subaxillary and suprainguinal region of each flank. Additional streaks of Wnt10b expression in the axillary and inguinal region join the mammary line on the flank slightly later. Expression of Wnt10b and Wnt6 becomes restricted to the placodes within 1.5 days. The ectoderm of the mammary line is organized as a pseudostratified epithelium connecting the developing mammary placodes at around E11.5, whereas all other surface ectoderm is single-layered. Thus, the mammary line expressing Wnt10b defines a distinct ectodermal region that we consider the equivalent of the ectodermal ridge in, for example, rabbit. To date, the formation of the mammary line expressing Wnt10b is the earliest discernible ectodermal event in murine embryonic mammary gland development. Developmental Dynamics 229:349–356, 2004. © 2004 Wiley-Liss, Inc.