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The formation and development of the ostrich sperm tail follows the general pattern described for vertebrate sperm but differs in a number of important aspects when compared with other nonpasserine birds. Midpiece formation is characterised by the development of a long (3 microns) distal centriole which determines the length of the midpiece. The distal centriole surrounds a pair of microtubules embedded in a core of dense material. A prominent annulus, which remains stationary during spermiogenesis, demarcates the midpiece/principal piece junction. Ostrich spermatids display similar cytoplasmic densities (chromatoid body, annulus, granulated bodies, reticulated bodies and mitochondria-associated material) to those observed in mammalian spermatids. In the ostrich, however, the chromatoid body is not observed to contribute towards the formation of the annulus while the granulated bodies develop within radial bodies and not in their vicinity as in mammals. Whereas in mammals granulated bodies have been linked to coarse fibre formation they appear to perform a limited function in ostrich spermatids as only rudimentary outer dense fibres are formed. No specific role is attributed to the reticulated bodies but the mitochondria associated material is implicated in the formation of intermitochondrial cement. The formation of the principal piece and end piece of the flagellum is modelled on that of the mammalian spermatid. The axoneme emerges from the free end of the distal centriole and the ribs and longitudinal columns are progressively formed from a layer of amorphous material situated between the axoneme and the plasmalemma. The end piece is formed by an extension of the principal piece consisting of axonemal microtubules closely surrounded by the plasmalemma. The formation of a long distal centriole containing a central pair of microtubules, the stationary nature of the annulus and the presence of a ribbed fibrous sheath around the principal piece distinguishes spermiogenesis in the ostrich and rhea from that of other nonpasserine birds. These features appear to be characteristic of more primitive avian families.