Some aspects of the life history of the Malaysian strain of Plasmodium juxtanucleare are presented. The periodicity of the parasite is believed to be an asynchronous, quotidian parasitemia with a long period of chronicity. Mature parasites are more common in visceral blood than in the peripheral circulation, indicating deep circulation schizogony. The parasite normally invades mature erythrocytes, with multiple infection of a cell uncommon except in instances of high parasitemia. The gametocytes are only slightly infective to mosquitoes during the first 30 days of patency, but subsequently become much more infective. Chronic infections remain highly infective to mosquitoes for as long as 198 days. The parasite could not be transmitted to canaries or young ducks. In the first paper of this series (Bennett and Warren, 1966) the asexual stages of the Malaysian strain of Plasmodium juxtanucleare were described, and in the second paper (Bennett et al., 1966) the development of the same parasite in the mosquito host was presented. The biology of P. juxtanucleare has been investigated to some extent by Al-Dabagh (1961) working with the Brazilian strain and by Dhanapala (1962) with the Ceylonese strain. This report is concerned with the biology of the Malaysian strain in the avian host with particular reference to periodicity of the asexual parasites in the peripheral blood, host cell preference, and infectivity of the gametocytes. In addition, this strain is compared with other known strains of the parasite and with other members of the subgenus Novyella. MATERIALS AND METHODS The parasite was maintained in the laboratory in young, hatchery-bred chickens by a combination of parasitized blood and sporozoite transfers. All birds were maintained in mosquito-proof cages. Subcutaneous, intravenous, and intraperitoneal routes were used for inoculations. Heparin was used as the anticoagulant for all parasitized blood transfers. Routine blood smears were made from the brachial vein at 0800 hr and stained with Giemsa stain at pH 7.2. Incisions through the body wall above the liver on anesthetized birds provided access to the liver for the hepatic smears. The incisions were closed with adhesive plaster and the birds treated with penicillin. The birds were then Received for publication 1 March 1966. * Present address: Ontario Research Foundation, 43 Queen's Park, Toronto 5, Ontario, Canada. anesthetized with ether, the incision opened, and smears made from the cut surface of the liver at desired intervals. Sporozoites for both routine and experimental transfers were obtained by dissecting the salivary glands from infected mosquitoes into Haye's saline. The glands were then crushed and the sporozoites collected in a mixture of 90% physiological saline10% serum (monkey or rabbit).