Summary The 8- and 9-day-old pre-erythrocytic schizonts of Plasmodium ovale are described from the liver of chimpanzees. The exo-erythrocytic schizonts found in liver biopsies taken from chimpanzees 15, 16, 17, 18 and 19 days after inoculation with sporozoites of P. ovale are described. These later schizonts show great variation in size and, while there is an overall increase in average size from 35 µ × 24 µ (17th day) to 64 µ × 46 µ (19th day), this increase is not continuous and the average size of the 15-day-forms is slightly greater than that of the 16- and 17-day-forms. On the 19th day, 20 schizonts out of 67 were burst, bursting or mature. No such forms were seen in biopsies on the 15th to 18th days after inoculation. It is concluded that this evidence provides support for the hypothesis of successive growing exo-erythrocytic generations of P. ovale.