The sexual plasticity of fish gonads declines after the sex-differentiation period; however, the plasticity of the germ cells themselves after this stage remains poorly understood. We characterized the sexual plasticity of gonial germ cells by transplanting them into sexually undifferentiated embryonic gonads in rainbow trout (Oncorhynchus mykiss). Spermatogonia or oogonia isolated from the meiotic gonads of vasa-green fluorescent protein (Gfp) gene transgenic trout were transplanted into the peritoneal cavity of newly hatched embryos of both sexes, and the behavior of the GFPlabeled donor cells was observed. The transplanted spermatogonia and oogonia migrated towards the recipient gonadal anlagen, and were subsequently incorporated into them. We also confirmed that the donor-derived gonial germ cells resumed gametogenesis in the recipient somatic microenvironment synchronously with the endogenous germ cells. Surprisingly, the donor-derived spermatogonia started to proliferate and differentiate into oocytes in female recipients. At 2 years post-transplantation, the eggs from mature female recipients were artificially inseminated with sperm from intact male rainbow trout. Normal, live offspring with the donor-derived haplotype were obtained. In addition, oogonia-derived sperm were produced in the male recipients. These donor-derived sperm were shown to be fully functional, as live offspring carrying GFP-labeled germ cells with the donor haplotype were obtained in the first filial (F1) generation. These findings indicate that rainbow trout pre-meiotic germ cells, which are likely to be spermatogonial or oogonial stem cells, possess a high level of sexual plasticity, and that the sexual differentiation of germ cells is controlled solely by the somatic microenvironment, rather than being cell autonomous.