Cinematographic observation with phase contrast microscopy was undertaken to elucidate a course of penetration of cultured cells by Toxoplasma gondii (RH strain). The results are summarized as follows : 1) The parasites which moved toward the cells showed active, irregular movements. No penetration was observed by less active or completely inactived parasites. 2) In the course of penetration of the cell membrane, the parasite attached itself to the cell membrane at a pole of its major axis. The attached portion of the parasite formed a rostrum with which it seemed to bore a small hole in the cell membrane through which it entered the cytoplasm. The hole or invagination must be smaller than the parasite because when passing through, the parasite was constricted. The time required for penetration of the cell averaged about 40 seconds. 3) Following the penetration, the parasites moved slowly in direction of the cell nucleus, but none were observed to penetrate the nuclei. 4) No common, definite cellular response was observed but, in general, the movement of the cytoplasmic granules surrounding the parasites decreased with time. 5) Escape of toxoplasma from the infected cell was observed, which was due to the movement of toxoplasma itself. Since the cell from which the toxoplasma escaped was found rather inactive, it is probable toxoplasma is highly sensitive to the nutritional condition of host cell
