bPlasmodium parasites employ posttranscriptional regulatory mechanisms as their life cycle transitions between host cell invasion and replication within both the mosquito vector and mammalian host. RNA binding proteins (RBPs) provide one mechanism for modulation of RNA function. To explore the role of Plasmodium RBPs during parasite replication, we searched for RBPs that might play a role during liver stage development, the parasite stage that exhibits the most extensive growth and replication. We identified a parasite ortholog of the Mei2 (Meiosis inhibited 2) RBP that is conserved among Plasmodium species (PlasMei2) and exclusively transcribed in liver stage parasites. Epitope-tagged Plasmodium yoelii PlasMei2 was expressed only during liver stage schizogony and showed an apparent granular cytoplasmic location. Knockout of PlasMei2 (plasmei2 ؊ ) in P. yoelii only affected late liver stage development. The P. yoelii plasmei2 ؊ liver stage size increased progressively until late in development, similar to wild-type parasite development. However, P. yoelii plasmei2 ؊ liver stage schizonts exhibited an abnormal DNA segregation phenotype and failed to form exoerythrocytic merozoites. Consequently the cellular integrity of P. yoelii plasmei2 ؊ liver stages became increasingly compromised late in development and the majority of P. yoelii plasmei2 ؊ underwent cell death by the time wild-type liver stages mature and release merozoites. This resulted in a complete block of P. yoelii plasmei2 Malaria is caused by unicellular eukaryotic parasites of the genus Plasmodium. The life cycles of the Plasmodium parasites that cause human disease (including the major human parasite Plasmodium falciparum) and other mammalian parasites such as rodent malaria parasites (including P. berghei and P. yoelii) are similar. The parasites progress through an elaborate life cycle, part within an anopheline mosquito vector and part within the mammalian host. Within the host, the sporozoite stage first infects hepatocytes, within which the parasite then develops as a liver stage (also known as an exoerythrocytic form), an acyclic process that occurs once during a malaria infection. Intrahepatocytic parasite development progresses in a cellular process known as schizogony, an endomitotic process characterized by an enormous increase in cell mass with concomitant DNA replication and organelle replication but without cytokinesis. It is only in the final hours of liver stage schizogony that organelles and DNA completely segregate and are apportioned into tens of thousands of rapidly differentiating exoerythrocytic invasive merozoites. These are released and initiate the infection of erythrocytes. Erythrocytic schizogony is cyclic and generates approximately 20 to 30 new merozoites in each cycle. Parasite schizogony and merozoite formation in both hepatocytes and erythrocytes likely employ shared gene regulatory networks and cell regulatory mechanisms, but it is possible that because exoerythrocytic schizogony exceeds erythrocytic schizogony by m...