Female zebrafish have a prolific reproductive capacity, suggesting that a germline stem cell (GSC) population drives oocyte production. However, a zebrafish female GSC population has yet to be identified. Adult stem cells are defined by their ability to both self-renew and differentiate, and by their localization to a stem cell niche. We show here that mitotic and early meiotic germ cells are present in the adult ovary and that the zebrafish homolog of the conserved vertebrate GSC marker, nanos2, is expressed in a subset of pre-meiotic oogonia in the adult gonad. We propose that these nanos2+ cells are GSCs. Importantly, we find that mitotic, nanos2+, and early meiotic germ cells localize to the germinal zone, thus identifying this region as the probable ovarian GSC niche in zebrafish. nanos3, which encodes a conserved RNA-binding protein, is known to be required for the continued production of oocytes in the zebrafish. Although mammalian homologs of nanos3 are expressed in early spermatogonia, no study has defined the role of nanos3 in the regulation of vertebrate GSCs. Here we demonstrate that nanos3 function is required for the maintenance of GSCs, but not for their specification, and propose that nanos2 and nanos3 are partially redundant in this role.