Toxoplasma ‘Nicolle et Manceaux’ has been examined in the electron microscope in thin sections of infected tissue cultures. The intracellular forms, single or in rosette formations, show that the parasite has no flagella, cilia or other organelles for locomotion. It is covered by a double membrane which is especially well denned at the two poles. A ring-like formation, often slightly protruding, can be seen at one end of the parasite, and in connexion with this, long, homogeneous, dark-stained inclusions of still unknown nature. At the opposite pole a distinct opening has been found.

Mitochondria and strands of an endoplasmic reticulum are also present in the cytoplasm and a fine striation in the region of the periplast. No dividing forms have been observed. Near the nucleus, in the region of what may correspond to the centrosome and the Golgi apparatus, a few extremely fine granules and a spiral-like formation of fine striae or tubules have been observed.

This work has been partly supported by the National Research Council of Brazil. The cost of reproduction of figures was defrayed by the Institute de Biofísica da Universidade do Brasil.