From Ixodes ricinus ticks collected in a tick-borne encephalitis focus in Baden-Württemberg, an agent pathogenic exclusively for suckling mice was isolated. Obviously a virus, it passed membrane filters with a pore width of 200 nm. It was resistant to ether and sodium deoxycholate, but not to chloroform. In the complement fixation test it showed a close, and in the neutralisation test a more one-sided relationship to Colorado tick fever virus.