Of 55 flies developing from blastoderms which had received male or female pole cell transplants, 15 (7 females and 8 males) wer shown by progeny testing to be germ line chimeras. Since donor and host pole cells were genetically marked with contrasting X- or Y-linked alleles, the progeny testing scheme enabled the genotypic sex of the donor component undergoing gametogenesis to be identified as either the same as ('homosexual' chimeras) or opposite ('heterosexual' chimeras) that of the host. All seven of the female chimeras were identified as 'homosexual' chimeras carrying only chromosomally female XYX donor and XX host germ cells. Similarly, all eight males were shown to be 'homosexual' chimeras with chromosomally male XY donor and XY host germ cells. The chromosomal sex of the donor component undergoing gametogenesis was in every case the same as the phenotypic sex of the host. Since there is an equal probability of constructing either a 'homosexual' or a 'heterosexual' chimera during pole cell transplantation, the ability of pole cells to differentiate functional gametes in hosts of the opposite sex was tested 50% of the time even if sex reversal of these donor pole cells could not be demonstrated. Thus the absence of 'heterosexual' chimerism strongly supports the interpretation that the phenotypic sex of a germ cell in Drosophila is determined entirely by its own chromosome constitution, not by that of the gonadal mesoderm.