The relapsing malaria parasites are characterized by the production of sporozoites with varying potential for exoerythrocytic development. Some sporozoites develop soon after introduction to produce mature schizonts and merozoites that initiate the erythrocytic stage infection. Relapsing hypnozoite forms are characteristic of some strains of Plasmodium vivax and are more apt to develop late than early with many time intervals in between. Studies in Saimiri monkeys suggest another type of sporozoite-induced infection. With the Salvador I strain of P. vivax, early developing exoerythrocytic schizonts apparently release parasites with different levels of virulence for these monkeys, ranging from those producing high-level parasitemia to a more abundant avirulent form. The induction of low-density avirulent infections requires the development of more sensitive detection methods for the evaluation of sporozoite vaccines.