Ca++ was shown to be indispensable for the normal growth of cultures of Plasmodium falciparum. Inclusion of ethyleneglycolbis (β-amino-ethylether) N,N′-tetra-acetic acid (EGTA) caused blocking of the asexual cell cycle of the parasite in two sites, the first blockage occurring between 20 and 26 hours after invasion of the erythrocyte. It proved to be irreversible by additions of Mg++ or Ca++, and to lead to morphologically abnormal parasites arrested in the mature trophozoite stage of the cycle. The second site of inhibition was probably one of the steps in the process of invasion of the erythrocyte by the merozoite. When 1 mM EGTA was added 24–30 hours after the culture was synchronized the cell cycle of the parasite continued without any interference in the normal maturation until the development of schizonts and release of merozoites into the medium. However, reinvasion of fresh erythrocytes by these merozoites was impeded. The inhibition of reinvasion caused by EGTA was overcome by the addition of an excess of Ca++ but not by an excess of Mg++. After the addition of Ca++ to cultures blocked just before the invasion phase as schizonts, the merozoites were again rendered fully infective and the rate of invasion was similar to that in an untreated control culture. Implications of the effects of Ca++ depletion on the asexual cell cycle and possible applications of EGTA as a reversible inhibitor of the invasion process are discussed.