The importance of ticks as vectors of animal diseases of great economic significance has long been acknowledged. The role of ticks in veterinary medicine and the use of DDT in their control is discussed fully in Chapter VIII (Knipling). More recently, the role that ticks play in the transmission of human disease has been increasingly recognized. Among the diseases vectored by these arthropods are Rocky Mountain spotted fever and related rickettsioses, ‘Q’ fever, Bullis fever, tick-borne relapsing fever, Colorado tick fever, Russian spring-summer encephalitis, louping ill, tularemia, and perhaps, on infrequent occasions, western equine encephalomyelitis and plague (Craig and Faust [119]). A list of the species of ticks which have been found naturally infected with the agents of these diseases is formidable. The virus of Rocky Mountain spotted fever is vectored by 5 species of Dermacentor ticks, 4 species of Amblyomma, 2 species of Haemaphysalis, and 1 species each of Rhipicephalus, Ixodes, and Hyalomma. About a dozen species of Ornithodoros have been found responsible for the transmission of tick-borne relapsing fever. ‘Q’ fever can result from the bite of species of Dermacentor, Amblyomma, Ornithodoros, Rhipicephalus, Ixodes, Haemaphysalis, Hyalomma, and Otobius ticks. Dermacentor andersoni and perhaps D. variabilis harbor the agents of Colorado tick fever and, along with certain other Dermacentor species, those of tularemia. The lone star tick, Amblyomma americanum, transmits Bullis fever.