In 1958 a virus transmitted by a mosquito was isolated in the Slovakian village of Tahyna. Since this virus was unknown in Europe, a detailed study was begun. It was shown that this virus belonged to the California group of arboviruses and occurs in practically all European countries. It could very well be that young hares are the hosts for this virus, and that it multiplies in them. Foals and suckling pigs are also hosts and possibly also increase the virus. In patients observed so far, infection with the Tahyna virus appears with influenza-like symptoms. In some cases, meningoencephalitis and atypical pneumonia were observed.