The liver is the first organ infected by Plasmodium sporozoites during malaria infection. In the infected hepatocytes, sporozoites undergo a complex developmental program to eventually generate hepatic merozoites that are released into the bloodstream in membrane-bound vesicles termed merosomes. Parasites blocked at an early developmental stage inside hepatocytes elicit a protective host immune response, making them attractive targets in the effort to develop a pre-erythrocytic stage vaccine. Here, we generated parasites blocked at a late developmental stage inside hepatocytes by conditionally disrupting the Plasmodium berghei cGMP-dependent protein kinase in sporozoites. Mutant sporozoites are able to invade hepatocytes and undergo intracellular development. However, they remain blocked as late liver stages that do not release merosomes into the medium. These late arrested liver stages induce protection in immunized animals. This suggests that, similar to the well studied early liver stages, late stage liver stages too can confer protection from sporozoite challenge.Malaria is among the deadliest infectious diseases in the world. It is caused by protozoan parasites of the genus Plasmodium that undergo a complex life cycle in the mammalian host and the mosquito vector. A human malaria infection begins when a Plasmodium sporozoite delivered through the bite of an infected mosquito infects a hepatocyte in the host liver. Within an intrahepatic membrane-bound vacuole the sporozoite undergoes extensive physical transformation followed by nuclear divisions, cytoplasmic segmentation, and eventually the formation of thousands of merozoites (1). Merozoites exit the infected hepatocyte by budding off in membrane-bound vesicles termed merosomes (2). Merosomes extrude from the infected hepatocyte through the endothelial cell layer and are released into the neighboring sinusoids. Thus, hepatic merozoites are delivered directly into the blood stream where they initiate invasion of erythrocytes and the symptomatic phase of a malaria infection (2). Unlike other stages of the Plasmodium life cycle, the stages that develop inside the hepatocytes, called "liver stages" (LSs), 3 are relatively poorly understood. Although the execution of the LS developmental program must require a large repertoire of molecules, only a few have been functionally identified so far (3-10). LS are of significant clinical and biological interest. Inhibiting the growth of LS could prevent the pathology associated with the erythrocytic stages of a malaria infection. The morbidity associated with Plasmodium vivax, the major human species in South America and South Asia, partly results from its ability to form dormant liver stages, termed hypnozoites, against which there are few effective treatment options (11). Reactivated hypnozoites can cause disease relapse up to a year after initial infection. Finally, LS have long been recognized to be ideal targets for developing a pre-erythrocytic stage malaria vaccine. Animals immunized with irradiated or genetica...