The importance of Toxoplasma gondii in human disease stimulated a number of electron microscope studies on the structure of this protozoan parasite. Gustafson et al. first studied the fine structure by means of thin sections in 1954. Many other papers havs subsequantly appeared. It is well known that Toxoplasma gondii has two stages in its life cycle-the proliferative forms and the cyst. The purpose of the electron microscopical work reported here was to study the fine structure of Toxoplasma gondii with recent techniques clarifying the correlation between the proliferative forms and cyst. RH strain and KM strain as proliferative forms on the one hand and Beverley strain as a cyst form of Toxoplasma gondii on the other hand were used throughout this study. 1) The conoid, toxoneme, nucleus, nucleolus, osmiophilic granules, mitochondria and vacuoles were found in RH strain as wsll as in KM strain and Beverley strain. 2) The endoplasmic reticulum was found in the cytoplasm of RH strain and KM strain. It was better developed in KM strain than in RH strain. 3) The outside contour of the organism of Beverley strain was somewhat irregular and toxoneme of this organism was better developed than in the other two strains. 4) Vacuoles were found in RH strain, KM strain and Beverley strain. Furthermore, tube-like bodies were observed in the vacuoles of the organism of RH strain. 5) In KM strain, two organisms of the same size were demonstrated in the leucocytes. It was presumed that they were products of longitudinal division.