Sheep embryos were examined with the electron microscope in order to characterize their organelles and the changes that occur during the preimplantation period. The first sign of differentiation of trophoblast cells was the appearance of junctions between external cells at the 16-cell stage. Nucleoli developed a granular component suggesting the synthesis of ribosomal RNA at the 16-cell stage also. Centrioles were seen as early as the 8-cell stage. Intracytoplasmic vesicles were present in large numbers in all cleavage stages but disappeared at blastulation. Mitochondria progressed from a very electron-dense hook- or U-shaped form with a few cristae to a cylindrical or spherical form of light density with many transverse cristae. Microvilli were not seen until the blastocyst stage and then only on the exterior surface of the trophoblast cells. Crystalloid or virus inclusions were not observed. It was concluded that the fine structure and developmental changes in the early sheep embryo are very similar to those of other mammalian species.