Colorado tick fever is the only tick-transmitted virus disease of man that is recognized in the Western Hemisphere. Little is known of its geographical distribution or the distribution of virus-carrying ticks. Knowledge of this disease is derived almost solely from work done in Colorado. Becker,1 from observations made in Colorado, described a clinical entity distinguished from Rocky Mountain spotted fever by the absence of skin eruption and by uniform recovery and called it Colorado tick fever. Florio and associates,2 at the University of Colorado Medical School, Denver, isolated a virus from the blood of patients by the inoculation of human volunteers and hamsters. In subsequent publications,3 they reported isolation of the virus from ticks, Dermacentor andersoni, collected near Denver, and from D. variabilis, collected on eastern Long Island, N. Y. Koprowski and Cox4 noted that adult dilute brown agouti (dba) mice were suitable for isolating virus.