Ultrastructural changes of developing spermatids and associated Sertoli cells during spermiogenesis in the kowari, Dasyuroides byrnei, were observed by transmission electron microscopy. The early round spermatid in the kowari showed an acrosomal vacuole containing sparsely distributed material. The acrosomal vacuole grew to some degree and then collapsed upon itself and decreased in size. After the diminution of the vacuole, flattening and condensation of the nucleus began. At this period, the manchette, nuclear ring and caudal plaque appeared and the ectoplasmic specialization of the Sertoli cell developed in association with the acrosomal region of the spermatid head. Microtubules of the manchette were arranged obliquely or perpendicular to the long axis of the spermatid. As the spermatid developed further, the nucleus displayed a horseshoe shape in cross section and was flattened in longitudinal section. The ectoplasmic specialization which was the most developed at this period appeared like horns protruding from the spermatid nucleus. Immediately before spermiation, the flattened aspect of the spermatid head contacted an apical process of the Sertoli cell. The Sertoli cell apical process extended the spermatid head into the lumen. Long tubulobulbar complexes appeared in the Sertoli cell, after the ectoplasmic specialization had dissociated.