Tick-borne encephalitis is the major european arbovirosis. It is observed in Soviet Union, Central and Western Europe and Scandinavian countries. In France, natural foci have been detected in Alsace (more specialy in Illkirch forest, near Strasbourg). In the United Kingdom exists a sheep encephalomyelitis (loupingill) caused by an arbovirus not much different from the tick-born encephalitis virus. The tick-born encephalitis virus is a flavivirus transmitted by tick bites. Tick is also a virus reservoir; it remains infected all along its life, through its metamorphosises and transmits the virus to its progeny. Vector-ticks belong to various species of a same genus; Ixodes persulcatus in Far-Eastern Soviet Union, Ixodes ricinus in other european foci. Small rodents (field-mouse, vole) are mainly the prime vertebrate hosts. Man is more often infected by the tick bite, more rarely by ingestion of new milk from infected domestic animals. Prognosis of the disease is under geographical variations: severe in Far-Eastern Russia and much milder in Central Europe. Natural foci are detected by serological surveys in patients recovering from neurologic diseases, human populations and in domestic animals. Natural foci beeing localized, isolation and identification of the virus from ticks and local rodents are tried. Extension and creation of new natural foci are caused by big mammalians and by migratory birds which carry over virus and vector ticks. Prophylaxis for people going to or living in registered natural foci, consists of avoiding ticks bites by wearing tight clothes and using repellents. Immunization by a recent vaccine produced in Austria from cultures on chicken embryo has given promising results.