Selection of population of Tacaribe virus strain 11573 lethal for mice was carried out by serial intracerebral passage of the virus in adult mice. Viral populations have been characterized by determination of virulence for suckling, weanling, and adult mice, and by histopathologic changes observed in brains of adult mice after intracerebral inoculation. Some of the virus preparations produced 80 to 90 per cent mortality after two or three intracerebral passages in adult mice and maintained this virulence for 1 to 3 passages, after which the virulence rapidly declined with subsequent passages. Clinical signs of infection in adult mice were manifested by a rough hair-coat, ventriflexed posture, diminished activity, increased excitability, flaccid hind-limb extension with progressive paralysis and death. Histologic examination revealed meningoencephalitis.