Field studies of Colorado tick fever (CTF) in small mammals in Rocky Mountain National Park (RMNP) in 1974 established that Eutamias minimus and Spermophilus lateralis were the most important hosts for CTF virus and were the source of virus for immature stages of the tick vector, Dermacentor andersoni. Other species (Peromyscus maniculatus, Spermophilus richardsonii, Eutamias umbrinus) are secondary hosts. The intensity of viral activity in rodents varied greatly from locality to locality. Highest rodent infection rates were found to occur in the Moraine Park area of RMNP. Lowest infection rates occurred above 3,290 meters in altitude at Rainbow Curve and on the tundra. The prevalence of infection in rodents was constant from April--July (5--6% of animals captured were viremic) and then declined to 1.7--2.5% in August and September coincident with a decline in nymphal tick ectoparasitism. Many animals were captured which were simultaneously viremic and antibody-positive. Under field conditions, neutralizing antibody seroconversion does not always occur.