Abstract: 15 min-24h after the administration of pyronaridine (6 mk/kg, ED50) to the mice infected with chloroquine-resistant strain of Plasmodium berghei, the morphological changes of the erythrocytic stages were observed under electron microscope. The action of the drug appeared to be primarily on the pellicular complexes in the tro-phozoites which showed swelling, and became whorled multilamellate. The modification developed progressively. Subsequently, mitochondria became swollen, endoplasmic reticulum disappeared, ribosomes blurred and nucleus became more electron-dense. Finally, small, disrupted, dead parasites were detected. In contrast, no significant ultrastructural changes could be found in most of the erythrocytic stages of the parasites after the chloroquine administration (400mg/kg) to the infected mice.