Coltiviruses have been isolated from ticks of the family Ixodidae and from rodents and humans. CTFV is endemic in northwestern North America, where it causes CTF, a human disease initially confused with a mild form of Rocky Mountain spotted fever, which is caused by Rickettsia rickettsii. The causative agent was isolated by Florio and others (3) in 1946 from human serum by injection into adult hamsters. The virus was adapted to egg and mice, and suckling mice became the routine isolation system for CTFV