The testes of some different orders of eutherian mammals were examined by conventional electron microscopy with respect to their pattern of spermiogenesis. In addition, some of the testes were studied by cytochemical methods for demonstration of certain nuclear proteins and of glycoproteins in the acrosome and the plasma membrane of spermatids. It was found that although the basic pattern of spermiogenesis was similar in all species studied, there were pronounced dissimilarities in the final shape of the spermatids. Differences were also observed in the timing of the differentiation of several organelles. The head of late spermatids and spermatozoa of Primates, Carnivora and Perissodactyla was cone-shaped, whereas in Artiodactyla and Lagomorpha it was flattened or paddle-shaped, and in Rodentia hook-shaped. The size and shape of the acrosome varied considerably between the orders, as did the length of the middle piece.