We examined, ultrastructurally, gonadoblastomas from two phenotypic females with a 46,XY and 46,XY/45, XO genotype, respectively. Ultrastructurally, germ cells of gonadoblastoma resembled immature gonocytes; it was not possible to classify them into exact developmental stages given for normal fetal testes or ovaries. In the dysgerminomatous part of the first tumor, germ cells showed ultrastructural signs of differentiation comparable to abortive spermatogenesis as seen in testicular but not ovarian dysgerminosa. The sex cord cells resembled either Sertoli, granulosa, or ovarian stromal cells. The stroma surrounding the epithelial nests resembled the ovarian stroma and contained interstitial cells like Leydig or ovarian hilar cells or both. The epithelial nests of gonadoblastoma were surrounded by laminar multilayered basement membranes. The hyaline masses inside the tumor nests were composed of the same material.