A mouse brain adapted strain of epizootic hemorrhagic disease (EHD) virus of deer was studied by ultrathin section and negative contrast electron microscopy. In sections of infected BHK-21 cells and neurons of newborn mouse brains, aggregates of virus particles, some appearing to be lacking their outer coats, were observed within membrane-bound vesicles. The development of virus particles was associated with intracytoplasmic viral matrices, consisting of moderately electron-dense granules. The viral particles measured about 58.8 nm in diameter in ultrathin sections and 62.3 nm in negative contrast preparations. Tubular structures, closely associated with viral matrices, were frequently found in infected cells. In one deer intracerebrally inoculated with virulent EHD virus, viral matrices, virus particles, and tubular structures, identical with those in infected BHK-21 cells and mouse neurons, were found in the cytoplasm of endothelial cells of blood vessels. The morphological details of EHD virus are compared with those of other viruses, and the differences in structure are discussed.