A strain of malaria parasite (termed the Donaldson strain) recovered from a man who had served as an Army officer in the West Pacific during World War II, is compared morphologically with two known strains of P. vivax, St. Elizabeth and Chesson, to determine the taxonomic relationship of the strain. In the measurement of areas of three corresponding stages of each of the three strains and of the erythrocytes containing these stages, in the counts of the numbers of merozoites per mature schizont, in the observations on the shape of growing trophozoites and on the shapes of the parasitized erythrocytes, as well as on the thick-film appearance of the strain, the Donaldson strain differs greatly from the two vivax strains. In all of these respects it resembles published descriptions of P. ovale. There are minor differences between the strain and descriptions given by some of the workers on P. ovale. Some of these differences may be readily attributed to a difference in staining technic.