In Aotus trivirgatus, the night monkey, schizonts and mature trophozoites of Plasmodium falciparum are concentrated in myocardium, adipose tissue, and skeletal muscle where parasitized red cells line the veins and block some capillaries. Electronmicroscopy reveals gross distortion of host red cells with numerous ultramicroscopic abnormalities of the red cell surface. Ultrastructural lesions of the red cell membrane have been demonstrated in cells infected with Plasmodium coatneyi and, as we have shown, in A. trivirgatus infected with P. falciparum. In infections with these parasites deep vascular schizogony is a prominent feature. The superficial alterations of the parasitized cells appear to be sites of attachment to venous endothelium and to other parasitized erythrocytes, thus explaining the paucity of circulating cells with maturing parasites in falciparum malaria.