A new virus, for which the name Matucare is proposed, was isolated from soft ticks, Ornithodoros boliviensis, collected in houses near San Joaquín, Bolivia (latitude 13°S; longitude, 65°W). The virus was lethal for newborn mice and hamsters but not for adult rodents of the same species. The agent multiplied in Vero cell cultures, producing cytopathic effect and plaques. Matucare virus was sensitive to heat, sodium desoxycholate, chloroform, and to acid pH of 5.5 or less; but 5-iododeoxyuridine did not inhibit multiplication, suggesting a ribonucleic-acid component. Particle size was estimated to be between 35 and 80 mµ by filtration. Complement-fixing antigens were obtained from brains of infected newborn animals, but no immunologic relation was found to any of the known murine viruses or arboviruses tested.