An electron microscopic study of Plasmodium ovale in humans supplied information on the fine structure of erythrocytic trophozoites, presegmenting schizonts and gametocytes. Ultrastructural features of P. ovale appeared quite similar to those described in other mammalian malarial parasites. However, some minor structural variations were observed. The parasite was a fairly compact cell of an oval shape. However, occasionally trophozoites had slender cytoplasmic protrusions. Ingestion of host cell cytoplasm occurred cytostomally. Nucleus contained a nucleolus-like dense region. The asexual parasites possessed acristate mitochrondria and were surrounded by a single-membraned pellicle in addition to a parasitophorous vacuole membrane. The gametocytes, however, possessed cristate mitochondria and were surrounded by a three-membraned pellicle in addition to a parasitophorous vacuole membrane.