Electron micrographs of ultra-thin sections of erythrocytes taken from two Liberian children ill with Plasmodium falciparum malaria show that the appliqué forms of this parasite are clearly within the host cell. The general fine structure of the parasites resembles that of other mammalian malaria parasites, as does the mode of ingestion of host cell material by pinocytosis. Granules of haemozoin were usually found in small vesicles pinched off from the large food vacuole. Scattered through the cytoplasm of infected red cells were narrow clear clefts bounded on each side by two unit membranes. These probably represent the Maurer's clefts seen in light microscopy. The surface of infected erythrocytes was notably distorted, a phenomenon which may have a bearing on the stickiness of the infected red cells in human falciparum malaria and the segregation of these cells in the capillaries. Many uninfected erythrocytes showed a multiple alveolar, blister-like abnormality of a portion of the cell membrane; this was not seen in otherwise comparable blood from a case of P. ovale infection.