The life cycle of malaria parasites in the mosquito vector is completed when the sporozoites infect the salivary gland and are ready to be injected into the vertebrate host. This paper describes the fine structure of the invasive process of mosquito salivary glands by malaria parasites. Plasmodium gallinaceum sporozoites start the invasion process by attaching to and crossing the basal lamina and then penetrating the host plasma membrane of the salivary cells. The penetration process appears to involve the formation of membrane junctions. Once inside the host cells, the sporozoites are seen within vacuoles attached by their anterior end to the vacuolar membrane. Mitochondria surround, and are closely associated with, the invading sporozoites. After the disruption of the membrane vacuole, the parasites traverse the cytoplasm, attach to, and invade the secretory cavity through the apical plasma membrane of the cells. Inside the secretory cavity, sporozoites are seen again inside vacuoles. Upon escaping from these vacuoles, sporozoites are positioned in parallel arrays forming large bundles attached by multilammelar membrane junctions. Several sporozoites are seen around and inside the secretory duct. Except for the penetration of the chitinous salivary duct, our observations have morphologically characterized the entire process of sporozoite passage through the salivary gland.