Spermiogenesis in the translocation heterozygote T (1; 2H) 25(20) y l 25/FM6 has been studied with the electron microscope and compared with that in wild type males. It appears that the genetic lesion in the male sterility mutant is associated primarily with a failure in differentiation of the head. In wild type flies, the spermatid nucleus assumes a conchoidal shape; chromatin accumulates along the convex surface. Adjacent to the concave surface a large bundle of microtubules runs parallel to the long axis of the spermatid. A single row of microtubules is juxtaposed against the convex surface of the head. As differentiation proceeds, the nucleus elongates, chromatin condenses, and the nucleus is compacted to a final diameter of about 0.3 µ. In the sterile mutant the spermatid nucleus has an irregular or wedge-shaped profile and no concavity is formed, nor is the bundle of microtubules observed. The row of microtubules, however, is usually present around the periphery. The change from lysine-rich to arginine-rich histone in mature wild type sperm does not occur in the sterile male. The substructure of the axial filament and mitochondrial derivatives, however, are similar to those in wild type.