Abstract Electron microscopic findings in the aortas of 10 mice infected with encephalomyocarditis (EMC) virus are described. Cellular necrosis was found mostly in the adventitia and occasionally in the smooth muscle cells closest to the adventitial tunic. Viral crystals were frequently found in association with the intracellular necrosis. Viral crystals were found in the adventitia of all 10 mice and in the smooth muscle cells of the media in some of the animals. The relationship of these findings to the production of aortitis or arteritis, which could also ultimately result in arteriosclerosis, is discussed.