Ixodes ricinus is the only one of six tick species reported from Finland found to carry viruses. It transmits tick-borne encephalitis and Uukuniemi viruses, and transmission of a Kemerovo group virus is suggested by antibody findings. Tick-borne encephalitis is common only in the SW-archipelago; it is also found near the SE-boarder but is very rare elsewhere. In contrast, Uukuniemi virus is common throughout the distribution area of I. ricinus, being commoner on the mainland than in the archipelago. Medium, large and small mammals participate in the cycle of tick-borne encephalitis virus, though birds are not excluded and apparently they act as host to the Uukuniemi virus. The encephalitis virus causes clinical illness in man, with cases limited mostly to the SW-archipelago; the disease is a relatively mild form of tick-borne encephalitis, with biphasic and meningeal symptoms dominating.