Ultrastructural changes of the human erythrocytes infected with Plasmodium ovale were studied, and differences of P. ovale from other human malarial parasites were discussed. Four characteristic morphological alterations were observed on the host cells: caveola-vesicle complex, excrescence, nodule, and cytoplasmic cleft. Caveola-vesicle complexes consisted of caveolae surrounded by vesicles in an alveolar fashion and were formed along the host cell plasmalemma. Similar complexes have been reported previously in P. vivax but not in P. falciparum and P. malariae. This complex probably corresponds to a Schüffner's dot. Excrescences similar to those observed by others on the host cell membrane in P. falciparum and P. malariae were observed also in P. ovale. The excrescences in P. ovale, as in P. falciparum, were limited to the erythrocytes infected by asexual parasites, whereas the excrescences in P. malariae have been demonstrated on erythrocytes with asexual parasites and gametocytes. Nodules were observed on the erythrocytes infected with asexual parasites of P. ovale. Structures such as the nodule have not been described previously in the erythrocytes infected with any other species of malarial parasite. Clefts within the cytoplasm of the host erythrocytes were present in P. ovale, as in all other malarial parasites.