The primordial germ cells and the gonadal mesoderm were mapped in the Drosophila embryo by analyzing the patterns of mosaicism in 'normal' and 'transformed' gynandromorphs. Relative to the adult cuticular markers the germ cells map as the posterior most structure, which coincides with their known location in the blastoderm embryo. These data support the hypothesis that the gynandromorph map reflects the real position of the primordia in the embryo. Since after the blastoderm stage the primordial germ cells migrate anteriorly these data also indicate that the map in fact corresponds to the blastoderm stage and not to a later stage of development. The genital disc maps at a single median primordium anterior and ventral to the germ cells, the gonadal mesoderm is located anterior to the genital disc and also forms a single median primordium on the ventral side of the embryo. The primordia for the genital disc and the gonadal mesoderm are unusually large in size, which presumably reflects some indeterminacy of the cell lineage leading to an 'expansion' of the map.