Summary1. Electron micrographs of tissue sections of chorioallantoic membrane infected with vaccinia virus were examined. Some cells were found to be almost filled with virus, others contained the virus particles grouped in single inclusions of varying sizes. Only a few virus particles were seen scattered throughout the cytoplasm of cells in certain areas, and in other areas extra-cellular virus was found in large numbers. 2. Small or medium sized inclusions were always located at the nuclear membrane and in contact with it. The inclusions consisted of masses of virus particles imbedded in a matrix which was dense in small inclusions and less dense in larger inclusions. The virus particles were ovoid and varied in length from 0.23 μ to 0.28 μ. Sectioned virus particles indicate that all have a dense outer wall, and some could be observed to have an internal structure. 3. The possible relationship of these observations to the growth cycle of vaccinia virus has been considered.