The structure and cytopathology of obligate intracellular bacteria belonging to families Rickettsiaceae and Chlamydiaceae and their interaction with eukaryotic host cells were compared in electron microscopic studies. "Rickettsia-like" and "chlamydia-like" types of organization of bacterial cells and their interaction with host cells are presented. The rickettsia-like type is characterized by short rod-shaped cells multiplying freely ( extravacuolarly ) in the cytoplasm or nucleoplasm of the host cell; the chlamydia-like type has spherical cells multiplying inside the cytoplasmic vacuole limited by the host membrane. The rickettsia-like type includes the genus Rickettsia and rod-shaped symbionts from genera Wolbachia and Symbiotes ; the chlamydia-like type falls into genera Chlamydia, Ehrlichia, Cowdria and Neorickettsia . The transitional types represented by Wolbachia persica (type 1), Coxiella and Rickettsiella (type 2) are also described. The possible evolutional relationships of the genera comprising both families are considered and their classification is proposed.