Ehrlichiosis is a rickettsial disease of animals and humans caused by various species of Ehrlichia. These obligate intracellular microorganisms infect granulocytes or monocytes of mammalian hosts and are transmitted mostly by ticks. The febrile disease is often undiagnosed and characterized by leukopenia, anemia and thrombocytopenia. The diagnosis of ehrlichiosis is based on clinical findings, tick infestation, direct or indirect detection of the agent, and serology. The role of native and imported ehrlichiae of animals in Switzerland is discussed.