A rodent model of sporozoite immunization against malaria based on a single immunizing dose is described. The duration of protective immunity was measured as a function of dose, and the results suggest that the percentage of protection follows a bimodal distribution. The first peak occurs between days 7-12 after immunization, while the second peak occurs at approximately 28 days. Although the percentage of protection declines steadily after the second peak, some immunity was detectable as long as 140 days after immunization with the higher doses.