Blood-stage malaria parasites in the vertebrate host can develop either into the asexual, multiplying forms, called schizonts, or into gametocytes, the sexual stages of the parasite. In the present work we studied the differentiation into asexual parasites or gametocytes of the progeny of single, isolated schizonts of the clone 3D7A of Plasmodium falciparum, using monoclonal antibodies specific for the sexual or asexual stages of the parasite. We observed that schizonts obtained from a continuous culture undergoing serial cycles of growth and dilution with fresh red blood cells produced either only gametocytes or only asexual parasites, showing a high degree of commitment to one or the other developmental pathway. The relative proportion of schizonts which produced gametocytes was very low at low parasite densities in culture, while at high parasite densities a much greater proportion of schizonts produced gametocytes. Nevertheless, at both low and high parasite densities individual schizonts were almost always fully committed to producing only gametocytes or only asexual parasites.