SUMMARY The structure of Ehrlichia canis in blood monocytes of a dog naturally infected with ehrlichiosis was studied by light and electron microscopy. By light microscopy, it was observed that inclusions (morula forms) in the cytoplasm of monocytes contained subunits. By electron microscopy, it was observed that an inclusion was surrounded by a single membrane and that it contained many organisms (elementary bodies) of various shapes. The organisms were surrounded by a double membrane and contained fine fibrils and granules.