Studies were done on the development of infectivity during ontogeny of the sporozoite of the rodent malaria parasite, Plasmodium berghei. Populations of sporozoites were separated from the oocysts, the hemocoel, and the salivary glands, with special precautions being taken to avoid cross-contamination between the different populations. The results indicated that populations of salivary gland sporozoites were more than 10,000 times as infective as populations of oocyst sporozoites from the same mosquitoes. The development of this infectivity appears to be asynchronous, in some cases taking place in the hemocoel, while in other cases not occurring until after the sporozoites have invaded the salivary glands. Thus, the development of infectivity seems to be time-dependent rather than site-dependent. There is also a continued increase in sporozoite infectivity during their residence in the salivary gland. The development of infectivity may be associated with other aspects of sporozoite maturation, including changes in their antigenicity and motility.