Piwi is necessary for germ stem cell survival in Drosophila and homologues have been identified in a diverse range of organisms. Here, we identify and characterize two homologous genes of piwi, Olpiwi1 and Olpiwi2, in the model fish medaka (Oryzias latipes). Olpiwi1 is similar to Ziwi in zebrafish or Miwi in the mouse, and Olpiwi2 is similar to Zili in zebrafish or Mili in the mouse. Moreover, Olpiwi2 mRNA is produced from two different chromosomes. RT-PCR showed expression of Olpiwi1 and Olpiwi2 predominantly in the gonads. In situ hybridization revealed germ cell-specific expression of Olpiwi1 and Olpiwi2 throughout the development of oocytes from oogonia to mature oocytes in the ovary, and from spermatogonia to spermatocytes in the testes of adults. RT-PCR and whole mount in situ hybridization showed that both Olpiwi1 and Olpiwi2 were maternally deposited in the embryo. Olpiwi1 and Olpiwi2 were detected in primordial germ cells during embryonic development. These results suggest that both Olpiwi1 and Olpiwi2 are germ cell specific, and may play important roles in germ cell development and gametogenesis in this model species. (C) 2011 Elsevier Inc. All rights reserved.