Fox12 is a member of the winged helix/forkhead family of transcription factors and is known to be involved in ovarian development in some vertebrates. To address the role of Fox12 in ovarian differentiation in medaka, we isolated Fox12 cDNA and analyzed its expression patterns during sex differentiation. Expression of Fox12 started in somatic cells Surrounding germ cells in XX gonads, just after initiation of ovarian differentiation, and was maintained in granulosa cells throughout ovarian development. In the adult ovary, Fox12 was expressed in previtellogenic and vitellogenic follicles, but expression ceased in postvitellogenic follicles. In contrast, Fox12 mRNA could not be detected in testes. In addition, Fox12 and aromatase mRNAs were co-localized in some somatic cells located on the ventral side of developing XX gonads. Our results suggested that Fox12 was not involved in ovarian determination, but was involved in differentiation of granulosa cells in medaka. (c) 2006 Elsevier Inc. All rights reserved.