Our analysis of spermiogenesis ofDrosophila subobscuraindicates that the axoneme takes part in the elongation of the spermatid nucleus, as follows. In sperm ofD. subobscurathe axoneme accompanies the nucleus in its full length up to the acrosome. Before the elongation of the nucleus begins, the centriole contacts the nuclear membrane, and is orientated with its axis to the centre of the spherical nucleus. Later in development, at the beginning of nucleus elongation, the axis of the centriole does no longer point to the centre of the nucleus but is dislocated more to one side of the nucleus. Subsequently, the axoneme which is growing from the centriole, pushes the nucleus which develops a cap-like structure over the anterior end of the centriole. By the continuosly growing axoneme stretching forces are applied to the anterior part of the nucleus. Consequently the elongating nucleus gets a smaller diameter anteriorly than posteriorly. And the longer the total length of the sperm is the longer is the nucleus. During elongation the chromatin shows a network-like structure. Nucleus elongation stops when the chromatin is fully condensed but the axoneme continues to grow. Thereupon the cap is no longer seen, and the anterior part of the nucleus which previously was the cap, forms now a bulge beside the centriole and the axoneme.