Fertile and diploid nuclear transplants were successfully generated by using embryonic cells as donors in a small laboratory fish, medaka (Oryzias latipes). Embryonic cell nuclei from transgenic fish carrying the green fluorescent protein (GFP) gene were transplanted into unfertilized eggs enucleated by x-ray irradiation. In this study, 1 out of 588 eggs transplanted in the first experiment and 5 out of 298 eggs transplanted in the second experiment reached the adult stage. All of these nuclear transplants were fertile and diploid, and the natural and GFP markers of the donor nuclei were transmitted to the F(1) and F(2) offspring in a Mendelian fashion. This systematic study proves the feasibility of generating nuclear transplants by using embryonic cells from fish as donors, and it is supported by convincing evidence.