The ultrastructural study of sporogony of Plasmodium b. berghei in the midgut of the mosquito Anopheles stephensi shows a process of multiple nuclear divisions. Several mitoses can be observed in the same section of the nucleus. These mitoses take place in the cortical nuclear zone, without disappearance of the nuclear envelope. The mitotic apparatus includes dense centers designated as kinetic centers (centres cinétiques) which are localized in the nuclear envelope. Early in mitosis, the kinetic center duplicates and a set of long and short microtubules is present on each kinetic center, short microtubules are in relation with cinetochores. A thin dense band of material links the two kinetic centers recalling the bridge observed in mitosis in the yeasts. The two halfspindles reorient so that the microtubules fall into line. Chromosomes are not individually visible and do not condense. Cinetochores are present; eight cinetochores can be observed at each halfspindle pole. All the divisions of the sporogony, except the last, are associated with cup-shaped invaginations of the nuclear envelope; the nuclear pores are abundant in the invaginations. The multiple nuclear division process allows the constitution of numerous genetic units in the nuclear cortex which would induce secondarily the sporozoite differentiation. Analogies with the mitosis in yeast Saccharomyces cerevisiae and the slime mold Dictyostelium discoideum are discussed.