The morphology of Cowdria ruminantium is described and its staining characteristics in the vertebrate host and in vitro are summarized. Morphologically, the organisms are characterized in the cytoplasm of endothelial cells, macrophages and reticulo-endothelial cells. Based on the morphology of the internal structure of the organisms, elementary (electron-dense), intermediate and reticulate bodies are identified. Each organism is surrounded by a double membrane and a "capsule" is evident around a few organisms in vitro. Usually, only organisms of the same form are found within a particular vacuole, although mixed colonies are described in the in vitro studies.