Three persons were experimentally inoculated with malaria by means of Anopheles ludlowi reared from larvae and infected with a pure strain of subtertian plasmodium (Plasmodium falciparum), thus proving that there exists no mechanical impediment or obstacle to the free exit of sporozoites from the salivary ducts or proboscis. In the dissection of infected mosquitoes there were no evidences of degenerated zygotes. Sporozoites appeared promptly in the salivary glands (9 to 12 days). Inoculation occurred with ease either in an interrupted feeding or after mosquitoes had been fed twice previously. The period of incubation was 14 and 18 days. The clinical manifestations were more severe in the subject that had never been infected with malaria previously, while the splenic enlargement was most pronounced in the subject infected after a long interval of freedom from malaria. In a third subject already suffering from tertian malaria there was only the slightest evidence of physical illness elicited by the superimposed subtertian infection; his temperature, however, became duly elevated. The type of febrile reaction in the two uncomplicated cases was at first tertian, becoming quotidian later, and this phenomenon in a pure strain leads strongly to the supposition that Plasmodium falciparum possesses inherently both tertian (or subtertian) and quotidian tendencies, as well as its well known tendencies to cause fever of the irregularly remittent or continued type. The creation of a specific plasmodium to account for clinical forms of aestivo-autumnal or subtertian malaria having a quotidian periodidty is probably unwarranted. In consideration of the facility with which this species can be infected and man inoculated experimentally, the occurrence of naturally infected wild specimens, and the positive epidemiological evidence, there should no longer exist in the minds of sanitarians any doubt as to its being a malarial carrier. Operations against this species can therefore be recommended without reservation and should be carried out without delay.