The morphologic characteristics of Eperythrozoon wenyoni were studied with the scanning electron microscope using blood from a naturally infected splenectomized Holstein-Friesan bull. Coccoid and oval organisms were seen in epicellular locations as solitary bodies and arranged in chains, clusters, or pairs. Budding seemed to be the primary mode of replication as structures resembling buds were encountered in association with some well-developed organisms. In some instances, the erythrocyte surface at the site of parasitic adherence seemed to be slightly depressed. Neither the erythrocyte surface membrane nor the morphology of the erythrocytes appeared significantly different from that of normal erythrocytes. The lack of notable erythrocyte membrane alterations was considered important in that anemia was not seen during or following parasitemia in this case. Eperythrozoon wenyoni, a red cell parasite of cattle, was first described by Adler and Ellenbogen (1934) in a splenectomized calf. The organisms in Giemsa-stained blood films appeared coccoid and vesicular in nature, either scattered regularly on erythrocytes, arranged in chains, or in tight groups. Neitz (1940) studied eperythrozoonosis in cattle and found delicate ring forms to be most common with ovoid, comma, rod, dumbell, and tennis racket forms also present. The parasites were seen both epicellularly and in the plasma. Lotze and Yiengst (1941) also observed similar organisms in cattle utilized in studies on anaplasmosis. A detailed study of the morphological characteristics of E. wenyoni was rendered by Kreier and Ristic (1963) using various techniques. Delicate basophilic rings, about 1 pu in diameter, were seen in the plasma and on erythrocytes in Giemsa-stained films, while spherical organisms were observed in wet preparations examined with the phase contrast microscope. Electron micrographs of shadow-cast preparations revealed the organisms to be oblong or circular in shape and attached to the erythrocyte surface, often in a peripheral position, or free in the plasma. E. wenyoni is encountered clinically in splenectomized cattle or in cattle severely ill from other causes (Schalm, 1965). Affected cattle may show signs of depression, fever, and sometimes of anemia. Parasitemia develops during the pyretic stage at which time paraReceived for publication 17 May 1973. sites can be found in great numbers on the red cells and in the plasma. The application of the scanning electron microscope in the study of erythrocytic parasites, both epicellular (Jain and Keeton, 1973) and intracellular (Keeton and Jain, 1973), has been fruitful in that certain heretofore unreported morphologic features of the parasites studied have been unveiled. The advantages of this mode of study are obvious, since a three-dimensional picture of an organism can be obtained. Erythrocytes parasitized with E. wenyoni from a splenectomized bull were examined utilizing the scanning electron microscope. The authors believe that this is the first report of such a study. MATERIALS AND METHODS Blood was collected by jugular venipuncture from a splenectomized yearling Holstein-Friesan bull naturally infected with E. wenyoni. Samples were obtained at the time of heavy parasitemia and 10 days later. Disodium ethylenediaminetetraacetate (2mg/ml) was used as the anticoagulant. Blood smears were prepared on cover slips and stained with Wright-Leishman or new methylene blue stains. The packed cell volume was determined by the microhematocrit method (Schalm, 1965). Erythrocytes were processed for scanning electron microscopy as described previously (Jain and Keeton, 1973). A second preparatory procedure (Bessis and De Boisfleury, 1970) which entailed double fixation was also employed. For this purpose, one drop of whole blood was placed into 10 ml of 1% glutaraldehyde in 0.1 M phosphate (pH 7.4) and allowed to fix for 1 hr. Thereafter, the cell suspension was centrifuged (1,500 g for 10 min), the supernatant decanted, and the pellet was resuspended in 2 ml of 1% osmium