The erythrocytes of 2 cats experimentally infected with Cytauxzoon felis were examined by light and electron microscopy. In stained blood smears, parasitized erythrocytes usually contained a single, roundish organism, but occasionally up to 4 were present in a cell. Chains of these roundish organisms also were seen. Elongated parasites, sometimes with ear-like projections, were present in a few erythrocytes. By electron microscopy, the parasite contained a poorly defined nucleus, rough endoplasmic reticulum, ribosomes, nonplicated mitochondria, food vacuoles, and a cytostome on its limiting membrane. Usually, the parasite was oval, but budding forms also were evident. Crystalloid inclusions were present in parasitized and nonparasitized erythrocytes.