In this paper a general review is given on some biological aspects of Toxoplasma gondii, especially on the ultraestructure of the interphasic form and the modifications which occur in theultrastructure during the division of the parasite. Initially the process of binary division has been discussed, admitting, however, that the images which have been considered as binary division might represent stages of the division by endodiogeny. The modifications are then described which occur in the fine structure of the "mother parasite" during the process of endodiogeny, i.e. the formation of the 2 "daughter parasites" in its interior. This process is the same in Toxoplasma gondii, Besnoitia jellisoni, Sarcocystis tenella and Frenkelia. The possibility is then discussed that "daughter parasites" are formed by the process of schizogony, as had been suggested by various authors. More recent results however show that schizogony does not exist in the vegetative forms of Toxoplasma gondii and that the images which were considered as such under the light microscope are the result of successive endodiogenies during which the newly formed endozoites remain united at their posterior region. Real schizogony, however, is found in the forms which develop in the interior of the epithelial cells in the intestine of the cat, which is considered the final host of Toxoplasma gondii. The concept of schizogony is then discussed, comparing this process in 3 different parasites as Eimeria bovis, Eimeria calospermophili and Plasmodium juxtanucleare which show certain differences in the initial phases of the individualization of the new parasites. The author then discusses the hypothesis which considers the endodiogeny the fundamental process of the division in Sporozoa, occuring at the final phase of schizogony. Finally, attention is given to the role which electron microscopy together with modem teehniques of cytochemistry and immunocytochemistry might play to reach a better understanding of the biology of Toxoplasma gondii and the phyiiopathology of toxoplasmosis.