In adult mammals, estrogen regulates ovarian function, and estrogen receptor (ER) is expressed in granulosa cells of antral follicles of the adult baboon ovary. Because the foundation of adult ovarian function is established in utero, the present study determined whether ERα and/or ERβ were expressed in fetal ovaries obtained on Days 100 (n = 3) and 165–181 (n = 5) of baboon gestation (term = Day 184). On Day 100, ERα protein was detected by immunocytochemistry in surface epithelium and mesenchymal-epithelial cells but not oocytes in germ cell cords. ERβ protein was also detected by immunocytochemistry on Day 100 of gestation and was abundantly expressed in mesenchymal-epithelial cells in germ cell cords, lightly expressed in the germ cells, but was not detected in the surface epithelium. On Days 165–180 of gestation, ERα expression was still intense in the surface epithelium, in mesenchymal-epithelial cells throughout the cortex, and in nests of cells between follicles. ERα expression was lighter in granulosa cells and was not observed in all granulosa cells, particularly in follicles close to the cortex. In contrast, ERβ expression was most intense in granulosa cells, especially in flattened granulosa cells, was weaker in mesenchymal-epithelial cells and nests of cells between follicles, and was absent in the surface epithelium. Using an antibody to the carboxy terminal of human ERβ, ERβ protein was also detected by Western immunoblot with molecular sizes of 55 and 63 kDa on Day 100 and primarily 55 kDa on Day 180. The mRNAs for ERα and ERβ were also detected by Northern blot analysis in the baboon fetal ovary. These results are the first to establish that the ERα and ERβ mRNAs and proteins are expressed and exhibit changes in localization in the primate fetal ovary between mid and late gestation. Because placental estrogen production and secretion into the baboon fetus increases markedly during advancing pregnancy, we propose that estrogen plays an integral role in programming fetal ovarian development in the primate.