An attenuated tissue culture adapted strain of DA virus, an agent related to the Theiler murine encephalomyelitis viruses (TMEV), was used to induce a chronic myelopathy in mice. Spastic paraparesis first appeared 5 months after weanings were inoculated intracerebrally with the virus. None died as a direct result of the infection, and none improved once paretic. The major pathological change in these mice was demyelination of thoracic segments of spinal cord. No clinical illness or demyelinative pathology were detected during the first 4 months after inoculation. Encephalitic virus was present in brain and spinal cord as late as 10 months after inoculation. No neutralization antibody activity to DA virus was present in sera from 10 patients with amyotrophic lateral sclerosis, 10 with multiple sclerosis, or in 10 controls.