The importance of Toxoplasma gondii in human systemic and eye disease has stimulated a number of electron microscopic studies on the structure and mode of division of this protozoan parasite. Gustafson et al13 first studied the fine structure by means of thin sections in 1954. Many other papers6,9,15,18,19,22,23,25,31 have subsequently appeared. In 1957 and 1958 Goldman et al10,11 pointed out that T. gondii proliferates by means of a peculiar mode of division, called endodyogeny, or internal budding. Although Gavin et al9 confirmed Goldman et al's light microscopic observations by means of electron microscopy, they suggested that additional forms of division included binary fission, as well as the sort present in schizogony. The purpose of the electron microscopical work reported here was to study the fine structure and mode of division of T. gondii with recently developed techniques, for correlation with the phase and fluorescent light microscopic