It has been possible to identify the chromosomes in electron micrographs of a prophase neuroblast nucleus inDrosophila melanogaster by using stereophotographs of stacked transparencies made from serial sections. Depth was determined by increasing or decreasing the stereo angle. Identification was facilitated by the use of a stock carrying chromosomal rearrangements. In this stock only chromosome 2 is metacentric. Compound chromosomes [symbolized C(3L)RM and C(3R)RM] were formed from the left and from the right arms of chromosome 3, and the X chromosome was intercalated in an inverted position between the two arms of the Y (YSX·YL).—Three-dimensional aspects of the ultrastructure of these chromosomes were observed as follows. The compacted centric regions are composed of fibers coiled in irregular gyres. In the extended regions, the fibers subdivide and appear uncoiled except for regions of compaction—“chromomeres”—which are seen in both homologues. Each homologue appears to be composed of four fibers of 130 A diameter, a total of eight for the prophase chromosome. Since the larger neuroblast cells are 8C in the G2 phase, it would seem that the 130 A fiber is equivalent to the cytological chromatid.