With the exception of the human malaria parasites, the mammalian species of Plasmodium have a fairly restricted geographical distribution, while the incidence of the infection in the different orders of mammals is patchy. A few species have been described in ungulates in the Old World Tropics, but Plasmodium odocoilei sp.nov. of the white-tailed deer (Odocoileus virginianus) of Texas is the first example to be found in the New World. It was discovered in an adult specimen that had had its spleen removed. Only the blood stages of the parasite are known, but these show various remarkable features: the presence of large vacuoles in the cytoplasm, and the enormous distortion and discoloration of the infected red blood cell. The phylogeny of the parasite is discussed in relation to that of the vertebrate host and it is suggested that Cervid stock crossed the Bering Land Bridge in the Pliocene Age, carrying a plasmodial infection that was the common source of malaria in the Old World Tragulidae and in the New World Odocoileus.