Plasmodium jefferyi is redescribed from the gibbon, Hylobates lar. Blood-induced infections may be severe with parasite counts up to 285,000 per mm' of blood. The periodicity is tertian. P. jefferyi can readily be separated from the other malarias in this host on morphological grounds, i.e., no increase in size of red blood cell, no stippling, older trophozoites frequently paisley-shaped, sixnucleate schizonts generally with eosinophilic ring, and reddish-pink staining of the mature microgametocyte with pigment concentrated to show stocking-cap effect. It is not infective to the rhesus monkey by inoculation of parasitized blood. The malaria parasite Plasmodium jefferyi Warren, Coatney, and Skinner, 1966, was first seen, in Kuala Lumpur, Malaysia, in July of 1964, as a low-grade natural infection in a juvenile gibbon, Hylobates lar, also infected with P. youngi. In the hope of obtaining a heavier infection of the then undescribed species, the infection was transferred to a malariafree gibbon, G 32, by the inoculation of parasitized blood. The original description of the parasite and its periodicity was based largely on material from that animal. Because the NIH-LPC installation at the Institute for Medical Research in Kuala Lumpur was to be abandoned shortly, the dual infection in G 32 was transferred to another gibbon (G 8) by the inoculation of parasitized blood. In September 1964, both animals were shipped to the LPC Section on Primate Malaria in Chamblee, Georgia (USA). Shortly after arrival in Chamblee, the parasite count in G 8 reach 102,000 per mm3 at which time blood was withdrawn and deepfrozen. Each of the host animals died during November 1964. The blood was left in the deepfreeze undisturbed until a power failure early in 1968 necessitated immediate action if the specimens were to be saved. At this juncture the senior Received for publication 10 June 1969. * This work was supported through a General Research Support Grant to LSU Medical School (FR-5376) and through Public Health Service Research Grant No. FR 00164 from the Division of Facilities and Resources, NIH. t Department of Parasitology, Delta Regional Primate Research Center, Covington, Louisiana 70433. + Laboratory of Parasite Chemotherapy, NIAID, NIH, Bethesda, Maryland 20014. author requested that the blood be sent to him in New Orleans, Louisiana. The parasitized blood was inoculated into a splenectomized, parasite-free gibbon, H. lar (G 420), at the Delta Regional Primate Research Center, Covington, Louisiana, in March 1968. The animal developed a patent infection 14 days later. However, it was quite different from the one in the donor animal (G 8) where P. youngi accounted for more than 80% of the parasite population at the time blood was drawn for freezing. In G 420 only one species of parasite was seen and, although it resembled P. jefferyi in many respects, other of its characteristics were quite different. For example, fully mature schizonts, not seen in previous P. jefferyi infections, were abundant in the peripheral blood as were distinctive male gametocytes. In addition, the infection appeared to be of a fulminating type. After intensive comparative study it became apparent that we were dealing with P. jefferyi under more favorable conditions than formerly. This being true, a corrected and expanded description of the species was required. What follows is a redescription of Plasmodium jefferyi based on material stained and examined under the same conditions as employed with the original material.