Royal Farm, a new group B virus from Argas hermanni ticks collected in Kabul Province, Afghanistan, is described. Its group relation through Powassan and Langat viruses is shown by complement fixation (CF) and neutralization tests (N). By CF test it is closely related to Powassan and by N test to Langat. Two isolations, 1 month apart, were made from ticks collected in pigeon shops in Kabul City and the Royal Farm in Kabul Province, respectively. The virus is pathogenic for hamsters but not for weanling guinea pigs and rabbits. Biologically, it differs from Powassan and Langat in its average survival time in 2-day-old mice, in its inability to kill adult mice by the intraperitoneal route, in having titers a minimum of 2 logs lower in suckling mice, and it kills chick embryos by the allantoic, amniotic, and yolk sac routes. Specific immune serum of Langat and Royal Farm reacted only with their homologous antigen in agar gel precipitation tests.