Thogoto (THO) virus infections of Rhipicephalus appendiculatus ticks were examined using tick hosts immune to the virus. In the first set of experiments, ticks were infected by feeding on viraemic hamsters. Inter-stadial infection of THO virus was not affected when ticks ingested a virus-immune bloodmeal but there was an effect on persistence of the virus. The incidence of intra-stadial infection was reduced by at least 40% when nymphs partially fed on viraemic hamsters and completed their bloodmeal on a virus-immune guinea pig. When the reverse situation was examined--feeding on a virus-immune host and then a viraemic host--no difference was observed in the number of ticks infected. In the second set of experiments, uninfected ticks acquired virus by co-feeding with infected ticks on apparently non-viraemic guinea pigs. Non-viraemic transmission of the viruses was inhibited when the guinea pigs were immune to either the Sicilian (SiAr 126) or prototype (IIA) isolates of THO virus. The laboratory data indicate that virus-immune hosts may have a significant effect on the role played by ticks in the epidemiology of tick-borne viruses.