Hygiene reports of medical officers of the US Army and trail diaries from a variety of sources contain convincing accounts of a milder form of mountain fever which are compatible with better-documented descriptions of Colorado tick fever (CTF). The virus of CTF is currently classified among the orbiviruses of the family Reoviridae. Orbiviruses are divided serologically into 12 groups. CTF is unique among orbiviruses containing 12 segments of double-strand Ribonucleic acid; all other orbiviruses contain 10 segments. CTF virus is the prototype of the CTF serogroup within the Orbivirus genus. More antigenic variation was found among strains of CTF isolated from humans than among those strains isolated from ticks or mammals. CTF virus can also be isolated from human or vertebrate blood clots or heparinized blood specimens. A few cases of CTF have been documented in pregnant women. From 1973 to 1986, the Division of Vector-Borne Viral Diseases collected information on 11 such patients.