Pregnancy-associated glycoproteins (PAGs) constitute a multigenic family of aspartic proteinases expressed in the trophoblast of the ruminant placenta. In Bos taurus, this family comprises 21 members segregated into ancient and modern phylogenetic groups. Ancient PAGs have been reported to be synthesized throughout the trophoblastic cell layer whereas modern PAGs are produced by binucleate cells of cotyledons. The aim of this study was to investigate modern and ancient PAGs during gestation in cotyledonary and intercotyledonary tissues. To obtain convincing and innovative results despite the high sequence identity shared between PAGs, we designed specific tools such as amplification primers and antibodies. Using real-time RT-PCR, we described the transcript expression of 16 bovine PAGs. Overall, PAGs are characterized by an increase in their expression during gestation. However, we demonstrated a segregation of modern PAGs in cotyledons and of ancient PAGs in the intercotyledonary chorion, except for the ancient PAG2 expressed in cotyledons. By raising specific antibodies against the modern PAG1 and ancient PAG11 and PAG2, we established the expression kinetics of the proteins using western blotting. Immunohistochemistry showed that PAGs were produced by specific cellular populations: PAG1 by binucleate cells in the whole trophoblastic layer, PAG11 was localized in binucleate cells of the intercotyledonary trophoblast and the chorionic plate of the cotyledon, while PAG2 was produced in mononucleate cells of the internal villi of the cotyledon. These results revealed a highly specific regulation of PAG expression and cell localization as a function of their phylogenetic status, suggesting distinct biological functions within placental tissues.